[gdb/testsuite] Fix duplicate test-names in gdb.ada

[gdb/testsuite] Fix duplicate test-names in gdb.ada

In gdb.ada we have these duplicate test-names:
...
DUPLICATE: gdb.ada/catch_ex.exp: continuing to program completion
DUPLICATE: gdb.ada/mi_catch_ex.exp: breakpoint at main
DUPLICATE: gdb.ada/mi_catch_ex.exp: mi runto main
DUPLICATE: gdb.ada/mi_catch_ex_hand.exp: breakpoint at main
DUPLICATE: gdb.ada/mi_catch_ex_hand.exp: mi runto main
...

Fix these using with_test_prefix.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-12  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/catch_ex.exp: Use with_test_prefix.
	* gdb.ada/mi_catch_ex.exp: Same.
	* gdb.ada/mi_catch_ex_hand.exp: Same.