tests: Fix few test for proper "skipped" detection with qemu-arm's tinytest.

"Builtin" tinytest-based testsuite as employed by qemu-arm (and now
generalized by me to be reusable for other targets) performs simplified
detection of skipped tests, it treats as such tests which raised SystemExit
(instead of checking got "SKIP" output). Consequently, each "SKIP" must
be accompanied by SystemExit (and conversely, SystemExit should not be
used if test is not skipped, which so far seems to be true).

tests/basics/builtin_compile.py

@@ -46,3 +46,4 @@ if have_compile():
     test()
 else:
     print("SKIP")
+    raise SystemExit

tests/basics/class_descriptor.py

@@ -27,8 +27,8 @@ except AttributeError:
 r = m.Forward
 if 'Descriptor' in repr(r.__class__):
     print('SKIP')
-else:
-    print(r)
-    m.Forward = 'a'
-    del m.Forward
+    raise SystemExit
 
+print(r)
+m.Forward = 'a'
+del m.Forward

tests/basics/fun_name.py

@@ -14,3 +14,4 @@ try:
     print(A().Fun.__name__)
 except AttributeError:
     print('SKIP')
+    raise SystemExit