android tests for an activity inside a directory

I am using Android Studio. I noticed the idea created a package (without being referenced in some manifest file or in the gradle build file). When I run test against my MainActivity, everything is going well, but when I want to run some test for an activity which is inside a package

I get this message :

java.lang.RuntimeException: Unable to resolve activity for: Intent { act=android.intent.action.MAIN flg=0x10000000 cmp=com.echopen.asso.echopen/.custom.CustomActivity }
at android.app.Instrumentation.startActivitySync(Instrumentation.java:384)
at android.test.InstrumentationTestCase.launchActivityWithIntent(InstrumentationTestCase.java:119)
at android.test.InstrumentationTestCase.launchActivity(InstrumentationTestCase.java:97)
at android.test.ActivityInstrumentationTestCase2.getActivity(ActivityInstrumentationTestCase2.java:104)
at com.echopen.asso.echopen.custom.CustomActivityTest.setUp(CustomActivityTest.java:19)