[gdb/testsuite] Make skip_libstdcxx_probe_tests return 1 if true

[gdb/testsuite] Make skip_libstdcxx_probe_tests return 1 if true

The tcl proc skip_libstdcxx_probe_tests currently returns 0 if the probe tests
need to be skipped, while tcl interprets 0 as false rather than true, which is
confusing.

Fix this by making skip_libstdcxx_probe_tests return 1 if the probe tests need
to be skipped.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-08-26  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (skip_libstdcxx_probe_tests_prompt): Return 1 if probe
	* tests need to be skipped.
	* gdb.cp/exceptprint.exp: Update call to skip_libstdcxx_probe_tests.
	* gdb.mi/mi-catch-cpp-exceptions.exp: Update call to
	mi_skip_libstdcxx_probe_tests.