test(tsdb): break up repeated test to avoid timeout (#10671)

On macOS, the TestTombstoneCleanRetentionLimitsRace performs very
poorly. It takes more than a second to write out one block, and as it
writes 400 of them, we run into the 10-minute test timeout frequently.

While this doesn't fix the actual performance issue, breaking each
iteration into a subtest makes the test pass reliably (because each
iteration comfortably finishes in under a minute).

Related report: https://groups.google.com/g/prometheus-developers/c/jxQ6Ayg6VJ4/m/03H_DS9PDAAJ

Signed-off-by: Matthias Rampke <matthias@prometheus.io>

tsdb/db_test.go

@@ -1281,6 +1281,7 @@ func TestTombstoneCleanRetentionLimitsRace(t *testing.T) {
 	//
 	// That is something tricky to trigger, so let's try several times just to make sure.
 	for i := 0; i < 20; i++ {
+		t.Run(fmt.Sprintf("iteration%d", i), func(t *testing.T) {
 			db := openTestDB(t, opts, nil)
 			totalBlocks := 20
 			dbDir := db.Dir()
@@ -1313,6 +1314,7 @@ func TestTombstoneCleanRetentionLimitsRace(t *testing.T) {
 			wg.Wait()
 
 			require.NoError(t, db.Close())
+		})
 	}
 }