Use concrete version of junit

mailbox/build.gradle

@@ -39,7 +39,7 @@ dependencies {
     kapt 'com.google.dagger:dagger-compiler:2.37'
     implementation 'androidx.appcompat:appcompat:1.0.0'
     implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
-    testImplementation 'junit:junit:4.+'
+    testImplementation 'junit:junit:4.13.1'
     androidTestImplementation 'androidx.test.ext:junit:1.1.1'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.0'
-}
\ No newline at end of file
+}