解决：Resolved versions for app (26.1.0) and test app (27.1.1) differ.
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.
解决方法：在app的build.gradle中添加androidTestCompile('com.android.support:support-annotations:26.1.0') { force = true }