gdb.base/setshow.exp: split in procs

gdb.base/setshow.exp: split in procs

Split in multiple procs, one per topic, and start with a fresh GDB in
each.  I find it easier to work on a test with multiple smaller
independent test procedures.  For example, it's possible to comment all
but one when working on one.  It's also easier to add things without
having to think about the impact on existing tests, and vice-versa.

Change-Id: I19691eed8f9bcb975b2eeff7577cac66251bcbe2