xdsclient/tests: move fallback tests to separate directory (#8600)

Currently, tests in the `internal/xds/xdsclient/tests` package can take
close to a minute to run. Almost half of that time is taken by the
fallback tests which actually have to run longer because they have to
wait for connections to go down and come up and for these events to be
detected by the code (before fallback is triggered).

Splitting the fallback tests into a separate directory almost reduces
the time by half since tests from these two packages can now run in
parallel.

We *could* possibly add a way for tests to add some dial options (to be
used when dialing the management server), and thereby reduce the time
spent in exponential backoff before connections are reattempted (during
the fallback process). But this would require non-trivial amount of
work, and could make the code more complicated. The change in this PR
seems like a good bang for the buck.

RELEASE NOTES: none

Author: easwars

internal/xds/xdsclient/tests/fallback_test.go renamed to internal/xds/xdsclient/tests/fallback/fallback_test.go

@@ -16,7 +16,7 @@
  *
  */
 
-package xdsclient_test
+package xdsclient_fallback_test
 
 import (
 	"context"
@@ -30,6 +30,7 @@ import (
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/internal"
+	"google.golang.org/grpc/internal/grpctest"
 	"google.golang.org/grpc/internal/stubserver"
 	"google.golang.org/grpc/internal/testutils"
 	"google.golang.org/grpc/internal/testutils/xds/e2e"
@@ -47,11 +48,23 @@ import (
 	v3discoverypb "github.com/envoyproxy/go-control-plane/envoy/service/discovery/v3"
 	testgrpc "google.golang.org/grpc/interop/grpc_testing"
 	testpb "google.golang.org/grpc/interop/grpc_testing"
+
+	_ "google.golang.org/grpc/internal/xds/httpfilter/router" // Register the router filter.
+	_ "google.golang.org/grpc/xds"                            // To ensure internal.NewXDSResolverWithConfigForTesting is set.
 )
 
+type s struct {
+	grpctest.Tester
+}
+
+func Test(t *testing.T) {
+	grpctest.RunSubTests(t, s{})
+}
+
 // Give the fallback tests additional time to complete because they need to
 // first identify failed connections before establishing new ones.
-const defaultFallbackTestTimeout = 2 * defaultTestTimeout
+const defaultFallbackTestTimeout = 20 * time.Second
+const defaultTestShortTimeout = 10 * time.Millisecond // For events expected to *not* happen.
 
 func waitForRPCsToReachBackend(ctx context.Context, client testgrpc.TestServiceClient, backend string) error {
 	var lastErr error