Refactor multiplatform test cases in MultiModuleHighlightingTest

Groupt multiplatform test cases in a nested class, get rid of duplicated
code, rename "header" -> "common" where it relates to the common module

idea/testData/multiModuleHighlighting/multiplatform/basic/common/common.kt

@@ -0,0 +1,3 @@
+header class <error descr="[HEADER_WITHOUT_IMPLEMENTATION] Header declaration 'My' has no implementation in module for JVM">My</error> {
+
+}

idea/testData/multiModuleHighlighting/multiplatform/basic/jvm/jvm.kt

@@ -0,0 +1,15 @@
+<error descr="[IMPLEMENTATION_WITHOUT_HEADER] Modifier 'impl' is only applicable to members that are initially declared in platform-independent code">impl</error> class Your {
+
+}
+
+header class <error descr="[HEADER_WITHOUT_IMPLEMENTATION] Header declaration 'His' has no implementation in module for JVM">His</error> {
+
+}
+
+header class Their {
+
+}
+
+impl class Their {
+
+}