解决 Resolved versions for app (26.1.0) and test app (27.1.1) differ 冲突依赖
Error:Execution failed for task ':app:preDebugAndroidTestBuild'.
> Conflict with dependency 'com.android.support:support-annotations' in project ':app'. Resolved versions for app (26.1.0) and test app (27.1.1) differ. See https://d.android.com/r/tools/test-apk-dependency-conflicts.html for details.
由上图可知，支持注解jar有两个，很明显冲突，可删除一个，方可解决，或者在build.gradle (app)中，添加如下
androidTestCompile('com.android.support:support-annotations:26.1.0') { force = true }