tests: Fix FST cleanup if alloc_fail is not supported

tests: Fix FST cleanup if alloc_fail is not supported

The test cases fst_ap_start_session_oom and fst_setup_mbie_diff did not
clean up FST sessions properly in case alloc_fail failed due to missing
support for it in the build. This could result in abandoning attached
hostapd global control interface monitors and test case failures due to
the global control interface socket running out of output buffer.

Fix this by going through the cleanup steps even if alloc_fail raises
HwsimSkip exception.

Signed-off-by: Jouni Malinen <j@w1.fi>