Add test which checks whether compiler messages are understood correctly by the plugin.

idea/testData/compilerMessages/helloWorld/src/test.kt

@@ -0,0 +1,4 @@
+package foo
+
+fun f() {
+}

idea/testData/compilerMessages/simpleError/src/test.kt

@@ -0,0 +1,5 @@
+package foo
+
+fun f() : Boolean {
+
+}

idea/testData/compilerMessages/simpleWarning/src/test.kt

@@ -0,0 +1,5 @@
+package foo
+
+fun f(s : String) {
+  s!!
+}