backupccl: deflake TestGCDropIndexSpanExpansion

With cockroachdb#73876 there is a bit more asynchrony than before and
thus the test must wait until all the ranges have completed splitting
before it attempts the last backup, so that the ExportRequest targets
the range with the correct SpanConfig applied to it.

Fixes: cockroachdb#75202

Release note: None

pkg/ccl/backupccl/backup_test.go

@@ -9007,11 +9007,12 @@ func TestGCDropIndexSpanExpansion(t *testing.T) {
 	}})
 	defer tc.Stopper().Stop(ctx)
 	sqlRunner := sqlutils.MakeSQLRunner(tc.Conns[0])
+	sqlRunner.Exec(t, `SET CLUSTER SETTING kv.closed_timestamp.target_duration = '100ms'`) // speeds up the test
 
 	sqlRunner.Exec(t, `
 CREATE DATABASE test; USE test;
 CREATE TABLE foo (id INT PRIMARY KEY, id2 INT, id3 INT, INDEX bar (id2), INDEX baz(id3));
-ALTER TABLE foo CONFIGURE ZONE USING gc.ttlseconds = '1';
+ALTER INDEX foo@bar CONFIGURE ZONE USING gc.ttlseconds = '1';
 INSERT INTO foo VALUES (1, 2, 3);
 DROP INDEX foo@bar;
 `)
@@ -9033,6 +9034,25 @@ DROP INDEX foo@bar;
 	// Wait for the GC to complete.
 	jobutils.WaitForJob(t, sqlRunner, gcJobID)
 
+	waitForTableSplit := func() {
+		testutils.SucceedsSoon(t, func() error {
+			count := 0
+			sqlRunner.QueryRow(t,
+				"SELECT count(*) "+
+					"FROM crdb_internal.ranges_no_leases "+
+					"WHERE table_name = $1 "+
+					"AND database_name = $2",
+				"foo", "test").Scan(&count)
+			if count == 0 {
+				return errors.New("waiting for table split")
+			}
+			return nil
+		})
+	}
+	waitForTableSplit()
+
+	// This backup should succeed since the spans being backed up have a default
+	// GC TTL of 25 hours.
 	sqlRunner.Exec(t, `BACKUP INTO LATEST IN 'nodelocal://0/foo' WITH revision_history`)
 }