[gdb/testsuite] Fix DUPLICATE in gdb.base/info-os.exp

When running test-case gdb.base/info-os.exp, I run into:
...
PASS: gdb.base/info-os.exp: get threads
PASS: gdb.base/info-os.exp: get threads
DUPLICATE: gdb.base/info-os.exp: get threads
...

Fix this not doing pass followed by exp_continue in gdb_test_multiple.

Tested on x86_64-linux.