[gdb/testsuite] Stabilize gdb-caching-proc.exp test order

[gdb/testsuite] Stabilize gdb-caching-proc.exp test order

The test-case gdb-caching-proc.exp tests each gdb_caching_proc in
gdb/testsuite/lib/*.exp.  However, the order of .exp file being tested can
change from run to run, because of using glob.

Fix this by sorting the glob result.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-08-21  Tom de Vries  <tdevries@suse.de>

	* gdb.base/gdb-caching-proc.exp: Sort files.