backupccl: deflake TestShowBackup

This patch updates the regex used in TestShowBackup to match on stringed
timestamps that contain any number of decimals after the second. I.e. both of
these timestamps match: "2023-09-21 01:13:25.3+00" and "2023-09-21
01:13:25.30+00".

Fixes: #111015

Release note: none

pkg/ccl/backupccl/show_test.go

@@ -124,7 +124,7 @@ ORDER BY object_type, object_name`, full)
 	// Truncate decimal places so Go's very rigid parsing will work.
 	// TODO(bardin): Consider using a third-party library for this, or some kind
 	// of time-freezing on the test cluster.
-	truncateBackupTimeRE := regexp.MustCompile(`^(.*\.[0-9]{2})[0-9]*\+00$`)
+	truncateBackupTimeRE := regexp.MustCompile(`^(.*\.\d{2})?[0-9]*\+00$`)
 	matchResult := truncateBackupTimeRE.FindStringSubmatch(beforeTS)
 	require.NotNil(t, matchResult, "%s does not match %s", beforeTS, truncateBackupTimeRE)
 	backupTime, err := time.Parse("2006-01-02 15:04:05.00", matchResult[1])