tests/basics: Move test for "return" outside function to own file.

Because its behaviour is conditional on MICROPY_CPYTHON_COMPAT.

tests/basics/syntaxerror.py

@@ -82,7 +82,6 @@ test_syntax("break")
 test_syntax("continue")
 
 # must be in a function
-test_syntax("return")
 test_syntax("yield")
 test_syntax("nonlocal a")
 test_syntax("await 1")

tests/basics/syntaxerror_return.py

@@ -0,0 +1,18 @@
+# With MICROPY_CPYTHON_COMPAT, the "return" statement can only appear in a
+# function.
+# Otherwise (in minimal builds), it ends execution of a module/class.
+
+try:
+    exec
+except NameError:
+    print("SKIP")
+    raise SystemExit
+
+try:
+    exec('return; print("this should not be executed.")')
+    # if we get here then MICROPY_CPYTHON_COMPAT is disabled and test
+    # should be skipped.
+    print("SKIP")
+    raise SystemExit
+except SyntaxError:
+    print('SyntaxError')