gdb: clear regcaches at the start of regcaches selftest

gdb: clear regcaches at the start of regcaches selftest

It currently does not work to run the `regcaches` selftest while
debugging something.  This is because we expect that there exists no
regcache at the start of the test.  If we are debugging something, there
might exist some regcaches.

Fix it by making the test clear regcaches at the start.

While at it, make the test clean up after it self and clear the
regcaches at the end too.

gdb/ChangeLog:

	* regcache.c (regcaches_test): Call registers_changed.

Change-Id: I9d4f83ecb0ff9721a71e2c5cbd19e6e6d4e6c30c