Fix TestCancelCompactions on windows

It seems that readOnlyDB was still opened which blocked the temp dir
cleanup.

Also changed the copy dir to be another TempDir instead of manually
creating one.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/compact_test.go

@@ -1171,12 +1171,9 @@ func TestCancelCompactions(t *testing.T) {
 	createBlock(t, tmpdir, genSeries(1, 1, 2000, 2001)) // The most recent block is ignored so can be e small one.
 
 	// Copy the db so we have an exact copy to compare compaction times.
-	tmpdirCopy := tmpdir + "Copy"
+	tmpdirCopy := t.TempDir()
 	err := fileutil.CopyDirs(tmpdir, tmpdirCopy)
 	require.NoError(t, err)
-	defer func() {
-		require.NoError(t, os.RemoveAll(tmpdirCopy))
-	}()
 
 	// Measure the compaction time without interrupting it.
 	var timeCompactionUninterrupted time.Duration
@@ -1228,6 +1225,7 @@ func TestCancelCompactions(t *testing.T) {
 		for i, b := range blocks {
 			require.Falsef(t, b.Meta().Compaction.Failed, "block %d (%s) should not be marked as compaction failed", i, b.Meta().ULID)
 		}
+		require.NoError(t, readOnlyDB.Close())
 	}
 }