Enable unit tests with Gradle

Summary:
This adds the same logic that we have in place with Litho for Yoga, allowing
unit tests to run with Gradle for Yoga. It'll still shell out to Buck to build
the native library, but that's hopefully only until we have found a way to reuse
the NDK build. For CI this shouldn't matter as we want to run `buck build`
anyway and the build step should be a no-op.

FB-Only:
This should unlock D5020044.

Reviewed By: emilsjolander

Differential Revision: D5104154

fbshipit-source-id: 48732fff6c1d100a155452f675de343aaf06427b

java/build.gradle

@@ -41,7 +41,10 @@ android {
             java.srcDir 'com'
             manifest.srcFile 'AndroidManifest.xml'
             res.srcDirs = ['res']
-            jniLibs.srcDirs = ['build/buck-out/jniLibs']
+        }
+
+        test {
+            java.srcDirs 'tests'
         }
     }
 }
@@ -50,6 +53,7 @@ dependencies {
     compile 'com.google.code.findbugs:jsr305:3.0.1'
     compile 'com.facebook.soloader:soloader:0.2.0'
     provided project(':yoga:proguard-annotations')
+    testCompile 'junit:junit:4.12'
 }
 
 task sourcesJar(type: Jar) {