Simplify tests on not-null assertions

No need to call blackBoxFile() on a useless test data file. Also creating
environment logic is simplified

compiler/testData/codegen/notNullAssertions/doGenerateAssertions.kt

@@ -1,4 +0,0 @@
-fun box(): String {
-    checkAssertions(true)
-    return "OK"
-}

compiler/testData/codegen/notNullAssertions/doNotGenerateAssertions.kt

@@ -1,4 +0,0 @@
-fun box(): String {
-    checkAssertions(false)
-    return "OK"
-}