错误内容：
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.
我们寻找External Libraries中的27.1.1版本，删掉就行了
最后在build.gradle中添加如下代码
androidTestCompile('com.android.support:support-annotations:26.1.0') {
force = true
}