Convert test_global, test_scope and test_grammar to unittest.

I tried to enclose all tests which must be run at the toplevel
(instead of inside a method) in exec statements.
diff --git a/Lib/test/README b/Lib/test/README
index 496c400..27f696c 100644
--- a/Lib/test/README
+++ b/Lib/test/README
@@ -379,8 +379,8 @@
       point numbers when you expect them to only be approximately equal
       withing a fuzz factor (``test_support.FUZZ``, which defaults to 1e-6).
 
-    * ``check_syntax(statement)`` - make sure that the statement is *not*
-      correct Python syntax.
+    * ``check_syntax_error(testcase, statement)`` - make sure that the
+      statement is *not* correct Python syntax.
 
 
 Python and C statement coverage results are currently available at