testing: move mockito from compile to testCompile

testing/build.gradle

@@ -4,7 +4,8 @@ dependencies {
     compile project(':grpc-core'),
             project(':grpc-stub'),
             libraries.junit
-    compile (libraries.mockito) {
+
+    testCompile (libraries.mockito) {
         // prefer 1.3 from JUnit instead of 1.1
         exclude group: 'org.hamcrest', module: 'hamcrest-core'
     }