roachtest: fix backup-restore compaction overlap bug

The previous implementation of the roachtest did not remove backup end
times from the list of candidate end times for compaction after
performing a compaction. This could result in scenarios where if
multiple compactions were attempted, it could choose a start time of a
backup that had been compacted. Because the compaction job compacts
backups from an elided backup chain, those compacted backups would not
appear in the list of backups and therefore the job would fail due to
not being able to find the starting backup.

Informs: #144216

Release note: None

Author: kev-cao

pkg/cmd/roachtest/tests/mixed_version_backup.go

@@ -15,6 +15,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"regexp"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -2076,7 +2077,7 @@ func (d *BackupRestoreTestDriver) createBackupCollection(
 		numIncrementals = 2
 	}
 	l.Printf("creating %d incremental backups", numIncrementals)
-	for i := 0; i < numIncrementals; i++ {
+	for i := range numIncrementals {
 		d.randomWait(l, rng)
 		if err := d.testUtils.runJobOnOneOf(ctx, l, incBackupSpec.Execute.Nodes, func() error {
 			var err error
@@ -2117,6 +2118,14 @@ func (d *BackupRestoreTestDriver) createBackupCollection(
 			}); err != nil {
 				return nil, err
 			}
+			// Since a compacted backup was made, then the backup end times of the
+			// backups it compacted should be removed from the slice. This prevents a
+			// scenario where a later compaction attempts to pick a start time from
+			// one of the backups that were compacted. Since compaction looks at the
+			// elided backup chain, these compacted backups are replaced by the
+			// compacted backup and compaction will fail due to being unable to find
+			// the starting backup.
+			backupEndTimes = slices.Delete(backupEndTimes, startIdx+1, endIdx)
 		}
 	}