vmtest: fix error reporting

S50-run-tests uses -e, which means that it immediately exits on test
failures without writing /exitcode. Fix by temporarily turning -e off.

Another issue is that $? in S50-run-tests is not quoted, which causes
the random value from the host to be taken (in practice always 0), so
fix that as well.

Finally, this fix has a positive side effect - QEMU no longer hangs
when tests fail. This is because rcS (generated by mkrootfs.sh) also
uses -e and immediately exits, if one of the scripts that it calls
fails, without calling S99-poweroff.

Example output after the fix:

Summary: 53/184 PASSED, 5 SKIPPED, 1 FAILED
+ exitstatus=1
+ set -e
+ echo 1
+ chmod 644 /exitstatus
+ for path in /etc/rcS.d/S*
+ '[' -x /etc/rcS.d/S99-poweroff ']'
+ /etc/rcS.d/S99-poweroff
travis_fold:start:shutdown
Shutdown

starting pid 232, tty '': '/sbin/swapoff -a'
starting pid 233, tty '': '/bin/umount -a -r'
[   45.909033] EXT4-fs (vda): re-mounted. Opts: (null)
The system is going down NOW!
Sent SIGTERM to all processes
Sent SIGKILL to all processes
Requesting system poweroff
[   48.932007] ACPI: Preparing to enter system sleep state S5
[   48.932785] reboot: Power down

Tests exit status: 1
travis_fold🔚shutdown

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/vmtest/run.sh

@@ -415,8 +415,8 @@ set -eux
 
 echo 'Running setup commands'
 %s
-%s
-echo $? > /exitstatus
+set +e; %s; exitstatus=\$?; set -e
+echo \$exitstatus > /exitstatus
 chmod 644 /exitstatus" "${setup_envvars}" "${setup_cmd}")
 fi