gdb/testsuite: clean ups in gdb.python/py-source-styling.exp

The top comment in gdb.python/py-source-styling.exp was completely
wrong, clearly a cut&paste job from elsewhere.  Write a comment that
actually reflects what the test does.

I've also moved the allow_python_tests check earlier in the file.

And I changed some 'return -1' into just 'return'.  I'm not aware that
the '-1' adds any value.

I also folded a 'pass $gdb_test_name' into the preceding gdb_assert,
which I think is neater.

There is no change in what is actually being tested after this commit.

Approved-By: Tom Tromey <tom@tromey.com>