Use test fixtures instead of sourceSets.test.output

This avoids the (often missing) evaluationDependsOn and fixes using
results from other projects without propagating those through
Configuration. It also reduces the number of useless classes pulled in
by down-stream tests, reducing the probability of rebuilds.

The expectation of fixtures is they help testing down-stream code that
use the classes in main. That applies to all the classes here except for
FakeClock and StaticTestingClassLoader. It would also apply to many
internal classes in grpc-testing, but let's consider cleaning that up
future work.
diff --git a/authz/build.gradle b/authz/build.gradle
index 2e6cfc1..1eb6c21 100644
--- a/authz/build.gradle
+++ b/authz/build.gradle
@@ -19,7 +19,7 @@
 
     testImplementation project(':grpc-testing'),
             project(':grpc-testing-proto'),
-            project(':grpc-core').sourceSets.test.output  // for FakeClock
+            testFixtures(project(':grpc-core'))
     testImplementation (libraries.guava.testlib) {
         exclude group: 'junit', module: 'junit'
     }