Improve versionbits_computeblockversion test code consistency

In this test, `nTime` is used for all the calls to `Mine()`, each time being set to the correct time beforehand, except for in the last few calls to `Mine()` where `nStartTime` is used directly, even though `nTime` is still set to `nStartTime` beforehand. `nTime` just remains unused for these last few calls to `Mine()`.

Changed the last few calls to `Mine()` to use `nTime` instead, improving consistency. This also fixes an unused value static analyzer warning about `nTime` being set to a value which is never used.

src/test/versionbits_tests.cpp

@@ -314,20 +314,20 @@ BOOST_AUTO_TEST_CASE(versionbits_computeblockversion)
 
     // Mine one period worth of blocks, and check that the bit will be on for the
     // next period.
-    lastBlock = secondChain.Mine(2016, nStartTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    lastBlock = secondChain.Mine(2016, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
     BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
 
     // Mine another period worth of blocks, signaling the new bit.
-    lastBlock = secondChain.Mine(4032, nStartTime, VERSIONBITS_TOP_BITS | (1<<bit)).Tip();
+    lastBlock = secondChain.Mine(4032, nTime, VERSIONBITS_TOP_BITS | (1<<bit)).Tip();
     // After one period of setting the bit on each block, it should have locked in.
     // We keep setting the bit for one more period though, until activation.
     BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
 
     // Now check that we keep mining the block until the end of this period, and
     // then stop at the beginning of the next period.
-    lastBlock = secondChain.Mine(6047, nStartTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    lastBlock = secondChain.Mine(6047, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
     BOOST_CHECK((ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit)) != 0);
-    lastBlock = secondChain.Mine(6048, nStartTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
+    lastBlock = secondChain.Mine(6048, nTime, VERSIONBITS_LAST_OLD_BLOCK_VERSION).Tip();
     BOOST_CHECK_EQUAL(ComputeBlockVersion(lastBlock, mainnetParams) & (1<<bit), 0);
 
     // Finally, verify that after a soft fork has activated, CBV no longer uses