[gdb/testsuite] Fix mi-catch-cpp-exceptions.exp and mi-nonstop.exp with check-read1

[gdb/testsuite] Fix mi-catch-cpp-exceptions.exp and mi-nonstop.exp with check-read1

With check-read1 we get:
...
FAIL: gdb.mi/mi-catch-cpp-exceptions.exp: check for stap probe in libstdc++
FAIL: gdb.mi/mi-nonstop.exp: probe for target remote
...

In both cases this is due to using gdb_test_multiple (which expects $gdb_prompt
by default) in combination with gdb using $gdb_mi_prompt, similar to the
problem fixed by commit d17725d72f "Don't expect gdb_prompt in
mi_skip_python_test".

Fix this by adding the $prompt_regexp argument to the gdb_test_multiple calls.

gdb/testsuite/ChangeLog:

2019-07-29  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (skip_libstdcxx_probe_tests_prompt, gdb_is_target_1):
	Pass prompt_regexp parameter to gdb_test_multiple calls.