gdb/testsuite: split gdb.python/py-parameter.exp in procs

gdb/testsuite: split gdb.python/py-parameter.exp in procs

Split the file into multiple independent test procs, where each proc
starts with a fresh GDB.  I find it easier to understand what a test is
doing when each part of the test is isolated and self-contained.  It
makes it easier to comment out some parts of the test while working /
debugging a specific part.  It also makes it easier to add new things
(which a subsequent patch will do) without fear of impacting another part
of the test.

Change-Id: I8b4d52ac82b1492d79b679e13914ed177d8a836d