Create new AbstractTransportTest for Transports

This is lower-level than the existing AbstractTransportTest. It should
be used by all transports, but InProcessTransport is the only one as
part of this commit.
diff --git a/core/build.gradle b/core/build.gradle
index c189b27..f7ccc82 100644
--- a/core/build.gradle
+++ b/core/build.gradle
@@ -7,6 +7,7 @@
 dependencies {
     compile libraries.guava,
             libraries.jsr305
+    testCompile project(':grpc-testing')
 }
 
 // Configure the animal sniffer plugin