Testsuite: Fix racy conditions in py-cmd.exp

Testsuite: Fix racy conditions in py-cmd.exp

The test loop in test_python_inline_or_multiline was not checking for the
gdb prompt. readline_is_used then reads in the prompt causing it to fail
and the final chunk of tests to be skipped.

Spotted this error because sometimes the gdb prompt does get processed
correctly and the final tests are run, causing it to show up in the
racy tests script output.

Fix by ensuring the prompt is always checked for. To do this the list
style needs reformatting so that variables get expanded.

2018-10-19  Alan Hayward  <alan.hayward@arm.com>

	* gdb.python/py-cmd.exp: Check for gdb_prompt.