[gdb/testsuite] Move gdb.base/gdb-caching-proc.exp to gdb.testsuite

Test-case gdb.base/gdb-caching-proc.exp doesn't really test gdb, but it tests
the gdb_caching_procs in the testsuite, so it belongs in gdb.testsuite rather
than gdb.base.

Move test-case gdb.base/gdb-caching-proc.exp to gdb.testsuite, renaming it to
gdb.testsuite/gdb-caching-proc-consistency.exp to not clash with
recently added gdb.testsuite/gdb-caching-proc.exp.

Tested on x86_64-linux.

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