[gdb/testsuite] Use with_test_prefix in gdb.base/gdb-caching-proc.exp

[gdb/testsuite] Use with_test_prefix in gdb.base/gdb-caching-proc.exp

When running test-case gdb.base/gdb-caching-proc.exp all passes are unique,
but fails might not be.

Fix this by using with_test_prefix.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-18  Tom de Vries  <tdevries@suse.de>

	* gdb.base/gdb-caching-proc.exp: Use with_test_prefix.