mixed builds: add cc_test bazel handler, and add adbd_test to staging allowlist.

This will allow the test_suites_{arm64, x86_64} CI targets on
git_master-bazel to build adbd_test with bazel in `m`.

This is currently implemented similar to the cc_binary handler to
hydrate the output path field.

The tradefed config is still currently handled by Soong in
testBinary#install, which is called from both ProcessBazelQueryResponse
and GenerateAndroidBuildActions. We'll move the tradefed config path to
a mixed builds handler when it's ready on the cc_test bazel rule side.

Tested with `atest` with cuttlefish. Since `atest` doesn't have a way to pass
--bazel-mode-staging to its internal call to Soong, so I manually tested
this by adding adbt_test to the prod allowlist, and confirmed that the
installed adbd_test is the same binary as the one in bazel-out.

Test: $ showcommands adbd_test | grep bazel-out | tail -n1

```
/bin/bash -c "rm -f
out/target/product/vsoc_x86_64/data/nativetest64/adbd_test/adbd_test &&
cp -f -d
out/bazel/output/execroot/__main__/bazel-out/android_x86_64_silvermont-opt-ST-59d4124ca041/bin/packages/modules/adb/adbd_test
out/target/product/vsoc_x86_64/data/nativetest64/adbd_test/adbd_test"
```

Test: Add adbd_test to prod allowlist and run 'lunch aosp_cf_x86_64_phone-userdebug; atest adbd_test', all tests pass.
Bug: 258426425
Bug: 261692372
Change-Id: I6be329f39d607af8daa1dc772b23e8dd0247418c
diff --git a/android/allowlists/allowlists.go b/android/allowlists/allowlists.go
index 06527bf..4cbeca5 100644
--- a/android/allowlists/allowlists.go
+++ b/android/allowlists/allowlists.go
@@ -1367,5 +1367,6 @@
 	// also be built - do not add them to this list.
 	StagingMixedBuildsEnabledList = []string{
 		"com.android.adbd",
+		"adbd_test",
 	}
 )
diff --git a/android/bazel_handler.go b/android/bazel_handler.go
index 0529f23..1c5860a 100644
--- a/android/bazel_handler.go
+++ b/android/bazel_handler.go
@@ -645,10 +645,12 @@
 
 mixed_build_root(name = "buildroot",
     deps = [%s],
+    testonly = True, # Unblocks testonly deps.
 )
 
 phony_root(name = "phonyroot",
     deps = [":buildroot"],
+    testonly = True, # Unblocks testonly deps.
 )
 `
 	configNodeFormatString := `
@@ -656,6 +658,7 @@
     arch = "%s",
     os = "%s",
     deps = [%s],
+    testonly = True, # Unblocks testonly deps.
 )
 `