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/headerPartiallyImplemented/common/common.kt

@@ -0,0 +1,23 @@
+header class <error>My</error> {
+
+    fun foo(): Int
+
+    fun bar(arg: Int): Boolean
+
+}
+
+header class <error>Your</error> {
+
+    fun foo(): Int
+
+    fun bar(arg: Int): Boolean
+
+}
+
+header class His {
+
+    fun foo(): Int
+
+    fun bar(arg: Int): Boolean
+
+}

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

@@ -0,0 +1,20 @@
+<error descr="[IMPLEMENTATION_WITHOUT_HEADER] Modifier 'impl' is only applicable to members that are initially declared in platform-independent code">impl</error> class My {
+
+    <error descr="[IMPLEMENTATION_WITHOUT_HEADER] Modifier 'impl' is only applicable to members that are initially declared in platform-independent code">impl</error> fun foo() = 42
+}
+
+<error descr="[IMPLEMENTATION_WITHOUT_HEADER] Modifier 'impl' is only applicable to members that are initially declared in platform-independent code">impl</error> class Your {
+
+    <error descr="[IMPLEMENTATION_WITHOUT_HEADER] Modifier 'impl' is only applicable to members that are initially declared in platform-independent code">impl</error> fun foo() = 13
+
+    <error descr="[IMPLEMENTATION_WITHOUT_HEADER] Modifier 'impl' is only applicable to members that are initially declared in platform-independent code">impl</error> fun bar(arg: Int) = arg
+
+}
+
+impl class His {
+
+    impl fun foo() = 7
+
+    impl fun bar(arg: Int) = arg == foo()
+
+}