gdb/testsuite: Fix minor bug in skip_btrace*tests procs

gdb/testsuite: Fix minor bug in skip_btrace*tests procs

The two guard functions skip_btrace_tests and skip_btrace_pt_tests
have a minor bug, if the check function fails to compile then surely
we should skip the btrace tests - currently we return 0 to indicate
don't skip.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (skip_btrace_tests): Return 1 if the test fails to
	compile.
	(skip_btrace_pt_tests): Likewise.

Change-Id: I6dfc04b4adcf5b9424fb542ece7ddbe751bee301