engine: Add more test cases to test mvccResolveIntentHistory

The previous change made mvccResolveIntentHistory more complex
in some subtle ways, such as the larger variety of cases where
they can expect to be called (such as around cleaning up intent
histories).

This change tests those cases and ensures that the resolution
code does the appropriate thing in all expected combinations of
those conditions.

Release note: None

pkg/storage/engine/mvcc.go

@@ -2751,8 +2751,6 @@ func mvccResolveWriteIntent(
 	// testing.
 	inProgress := !intent.Status.IsFinalized() && meta.Txn.Epoch >= intent.Txn.Epoch
 	pushed := inProgress && hlc.Timestamp(meta.Timestamp).Less(intent.Txn.WriteTimestamp)
-	collapsedIntent := false
-	var rolledBackVal []byte
 	latestKey := MVCCKey{Key: intent.Key, Timestamp: hlc.Timestamp(meta.Timestamp)}
 
 	// Handle partial txn rollbacks. If the current txn sequence
@@ -2761,7 +2759,9 @@ func mvccResolveWriteIntent(
 	// If _all_ the writes get removed in this way, the intent
 	// "collapses" and should be considered empty (i.e. can be removed altogether).
 	// If only part of the intent history was rolled back, but the intent still
-	// remains, updatedIntent is set to true.
+	// remains, the rolledBackVal is set to a non-nil value.
+	collapsedIntent := false
+	var rolledBackVal []byte
 	if len(intent.IgnoredSeqNums) > 0 {
 		collapsedIntent, rolledBackVal, err = mvccMaybeRewriteIntentHistory(ctx, rw, intent.IgnoredSeqNums, meta, latestKey)
 		if err != nil {
@@ -2791,7 +2791,7 @@ func mvccResolveWriteIntent(
 
 		// Update or remove the metadata key.
 		var metaKeySize, metaValSize int64
-		if pushed || (rolledBackVal != nil && !commit) {
+		if !commit {
 			// Keep existing intent if we're updating it. We keep the
 			// existing metadata instead of using the supplied intent meta
 			// to avoid overwriting a newer epoch (see comments above). The
@@ -2939,7 +2939,7 @@ func mvccResolveWriteIntent(
 // mvccMaybeRewriteIntentHistory rewrites the intent to reveal the latest
 // stored value, ignoring all values from the history that have an
 // ignored seqnum.
-// The cleared return value, when true, indicates that
+// The collapsed return value, when true, indicates that
 // all the writes in the intent are ignored and the intent should
 // not be considered to exist any more.
 // The updatedVal, when non-nil, indicates that the intent was updated
@@ -2950,7 +2950,7 @@ func mvccMaybeRewriteIntentHistory(
 	ignoredSeqNums []enginepb.IgnoredSeqNumRange,
 	meta *enginepb.MVCCMetadata,
 	latestKey MVCCKey,
-) (cleared bool, updatedVal []byte, err error) {
+) (collapsed bool, updatedVal []byte, err error) {
 	if !enginepb.TxnSeqIsIgnored(meta.Txn.Sequence, ignoredSeqNums) {
 		// The latest write was not ignored. Nothing to do here.  We'll
 		// proceed with the intent as usual.