test: getblockchaininfo projected activation_height test (#5665)

## Issue being fixed or feature implemented
https://github.com/dashpay/dash/issues/5640

## What was done?
Tests that `activation_height` projected by `getblockchaininfo` during
locked_in phase.
Now, this test is only possible with v20 activation since v19, dip0024
are buried and mn_rr uses MNEF.

Enabled this test only in `feature_llmq_rotation.py`.

## How Has This Been Tested?
tests

## Breaking Changes
no

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [x] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

---------

Co-authored-by: Konstantin Akimov <knstqq@gmail.com>
Co-authored-by: UdjinM6 <UdjinM6@users.noreply.github.com>

test/functional/feature_llmq_rotation.py

@@ -18,7 +18,7 @@ from test_framework.mininode import P2PInterface
 from test_framework.util import (
     assert_equal,
     assert_greater_than_or_equal,
-    wait_until,
+    wait_until, assert_greater_than, get_bip9_details,
 )
 
 
@@ -97,9 +97,18 @@ class LLMQQuorumRotationTest(DashTestFramework):
         expectedNew = [h_100_0, h_106_0, h_104_0, h_100_1, h_106_1, h_104_1]
         quorumList = self.test_getmnlistdiff_quorums(b_h_0, b_h_1, {}, expectedDeleted, expectedNew, testQuorumsCLSigs=False)
 
+        self.log.info(f"Wait for v20 locked_in phase")
+        # Expected locked_in phase starts at 1440 - 480 (window size in regtest)
+        projected_activation_height = self.advance_to_locked_in_for_v20(expected_locked_in_height=960)
+
         self.activate_v20(expected_activation_height=1440)
         self.log.info("Activated v20 at height:" + str(self.nodes[0].getblockcount()))
 
+        softfork_info = get_bip9_details(self.nodes[0], 'v20')
+        assert_equal(softfork_info['status'], 'active')
+        assert 'since' in softfork_info
+        assert_equal(projected_activation_height, softfork_info['since'])
+
         # v20 is active for the next block, not for the tip
         self.nodes[0].generate(1)
 
@@ -368,5 +377,51 @@ class LLMQQuorumRotationTest(DashTestFramework):
             return True
         return False
 
+    def advance_to_locked_in_for_v20(self, expected_locked_in_height):
+        # disable spork17 while mining blocks to activate "name" to prevent accidental quorum formation
+        spork17_value = self.nodes[0].spork('show')['SPORK_17_QUORUM_DKG_ENABLED']
+        self.bump_mocktime(1)
+        self.nodes[0].sporkupdate("SPORK_17_QUORUM_DKG_ENABLED", 4070908800)
+        self.wait_for_sporks_same()
+
+        # mine blocks in batches
+        batch_size = 10
+        height = self.nodes[0].getblockcount()
+        assert_greater_than(expected_locked_in_height, height)
+        # NOTE: getblockchaininfo shows softforks locked_in at block (window * 2 - 1)
+        # since it's returning whether a softwork is locked_in for the _next_ block.
+        # Hence the last block prior to the locked_in state is (expected_locked_in_height - 2).
+        while expected_locked_in_height - height - 2 >= batch_size:
+            self.bump_mocktime(batch_size)
+            self.nodes[0].generate(batch_size)
+            height += batch_size
+            self.sync_blocks()
+        blocks_left = expected_locked_in_height - height - 2
+        assert_greater_than(batch_size, blocks_left)
+        self.bump_mocktime(blocks_left)
+        self.nodes[0].generate(blocks_left)
+        self.sync_blocks()
+
+        softfork_info = get_bip9_details(self.nodes[0], 'v20')
+        assert_equal(softfork_info['status'], 'started')
+        assert 'activation_height' not in softfork_info
+
+        self.bump_mocktime(1)
+        self.nodes[0].generate(1)
+        self.sync_blocks()
+
+        softfork_info = get_bip9_details(self.nodes[0], 'v20')
+        assert_equal(softfork_info['status'], 'locked_in')
+        assert_equal(softfork_info['since'], expected_locked_in_height)
+        assert 'activation_height' in softfork_info
+        projected_activation_height = softfork_info['activation_height']
+
+        # revert spork17 changes
+        self.bump_mocktime(1)
+        self.nodes[0].sporkupdate("SPORK_17_QUORUM_DKG_ENABLED", spork17_value)
+        self.wait_for_sporks_same()
+
+        return projected_activation_height
+
 if __name__ == '__main__':
     LLMQQuorumRotationTest().main()