[gdb/testsuite] Fix gdb.base/stap-probe.exp with read1

When running test-case gdb.base/stap-probe.exp with make target check-read1, I
run into this and similar:
...
FAIL: gdb.base/stap-probe.exp: without semaphore, not optimized: \
  info probes stap (timeout)
...

Fix this by using gdb_test_lines instead of gdb_test.

Tested on x86_64-linux.