enhance tests (mostly codegen)

* drop dependency on stdlib/alt-headers in the most tests
* assign to myEnvironment at most once in JetLiteFixture
* more tests do not compile runtime

compiler/testData/codegen/classes/overloadPlusAssign.jet

@@ -12,7 +12,7 @@ class ArrayWrapper<T>() {
     }
 
     fun get(index: Int): T {
-        return contents.get(index)
+        return contents.get(index).sure()
     }
 }
 

compiler/testData/codegen/classes/overloadPlusAssignReturn.jet

@@ -15,7 +15,7 @@ class ArrayWrapper<T>() {
     }
 
     fun get(index: Int): T {
-        return contents.get(index)
+        return contents.get(index).sure()
     }
 }
 

compiler/testData/codegen/classes/overloadPlusToPlusAssign.jet

@@ -15,7 +15,7 @@ class ArrayWrapper<T>() {
     }
 
     fun get(index: Int): T {
-        return contents.get(index)
+        return contents.get(index).sure()
     }
 }
 

compiler/testData/codegen/classes/overloadUnaryOperator.jet

@@ -15,7 +15,7 @@ class ArrayWrapper<T>() {
     }
 
     fun get(index: Int): T {
-        return contents.get(index)
+        return contents.get(index).sure()
     }
 }