Remove unused JUnitMode.Mix mode

It's not used by now, and there's are known problems with it check
KTI-712.

If transition to JUnit5 is needed, creating another module might
be a better option.

buildSrc/src/main/kotlin/tasks.kt

@@ -88,8 +88,9 @@ fun Task.dependsOnKotlinGradlePluginPublish() {
     }
 }
 
+// Mixing JUnit4 and Junit5 in one module proved to be problematic, consider using separate modules instead
 enum class JUnitMode {
-    JUnit4, JUnit5, Mix
+    JUnit4, JUnit5
 }
 
 /**
@@ -142,19 +143,6 @@ fun Project.projectTest(
                     }
                 }
 
-                val parentNames = if (jUnitMode != JUnitMode.JUnit4) {
-                    /*
-                     * If we run test from inner test class with junit 5 we need
-                     * to include all containing classes of our class
-                     */
-                    val nestedNames = classFileNameWithoutExtension.split("$")
-                    mutableListOf(nestedNames.first()).also {
-                        for (s in nestedNames.subList(1, nestedNames.size)) {
-                            it += "${it.last()}\$$s"
-                        }
-                    }
-                } else emptyList()
-
                 include { treeElement ->
                     val path = treeElement.path
                     if (treeElement.isDirectory) {
@@ -162,11 +150,7 @@ fun Project.projectTest(
                     } else {
                         if (path == classFileName) return@include true
                         if (!path.endsWith(".class")) return@include false
-                        when (jUnitMode) {
-                            JUnitMode.JUnit4 -> path.startsWith("$classFileNameWithoutExtension$")
-                            JUnitMode.Mix -> parentNames.any { path.startsWith(it) } || path.startsWith("$classFileNameWithoutExtension$")
-                            else -> error("JUnit5 should not be reachable because of early exit")
-                        }
+                        path.startsWith("$classFileNameWithoutExtension$")
                     }
                 }
             }