tsdb/agent: use unique registry per tests

So tests can run in parallel.

Signed-off-by: Mateusz Gozdek <mgozdekof@gmail.com>

tsdb/agent/db_test.go

@@ -39,7 +39,7 @@ func TestUnsupported(t *testing.T) {
 	opts := DefaultOptions()
 	logger := log.NewNopLogger()
 
-	s, err := Open(logger, prometheus.DefaultRegisterer, nil, promAgentDir, opts)
+	s, err := Open(logger, prometheus.NewRegistry(), nil, promAgentDir, opts)
 	require.NoError(t, err)
 	defer func() {
 		require.NoError(t, s.Close())