test: improve debug-ability of test-execute

test: improve debug-ability of test-execute

Since e56a8790a0 debugging test-execute fails has been a royal PITA, since
we ditch all potentially useful output from the test units (that, for
the most part, run `sh -x ...`). Let's improve the situation a bit by
setting EXEC_OUTPUT_NULL only when running the single test case that
needs it, and inheriting stdout otherwise.

For example, with a purposefully introduced error we get this output
with this patch:
exec-personality-x86-64.service: About to execute: sh -x -c "c=\$\$(uname -m); test \"\$\$c\" = \"foo_bar\""
Serializing sd-executor-state to memfd.
...
        Personality: x86-64
        LockPersonality: no
        SystemCallErrorNumber: kill
++ uname -m
+ c=x86_64
+ test x86_64 = foo_bar
Received SIGCHLD from PID 1520588 (sh).
Child 1520588 (sh) died (code=exited, status=1/FAILURE)
exec-personality-x86-64.service: Child 1520588 belongs to exec-personality-x86-64.service.
exec-personality-x86-64.service: Main process exited, code=exited, status=1/FAILURE
exec-personality-x86-64.service: Failed with result 'exit-code'.
...
        Exit Status: 1
src/test/test-execute.c:456:test_exec_personality: exec-personality-x86-64.service: can_unshare=yes: exit status 1, expected 0
(test-execute-root) terminated by signal ABRT.
Assertion 'r >= 0' failed at src/test/test-execute.c:1433, function prepare_ns(). Aborting.
Aborted

But without it, we'd miss the most important part:
exec-personality-x86-64.service: About to execute: sh -x -c "c=\$\$(uname -m); test \"\$\$c\" = \"foo_bar\""
Serializing sd-executor-state to memfd.
...
        Personality: x86-64
        LockPersonality: no
        SystemCallErrorNumber: kill
Received SIGCHLD from PID 1521365 (sh).
Child 1521365 (sh) died (code=exited, status=1/FAILURE)
exec-personality-x86-64.service: Child 1521365 belongs to exec-personality-x86-64.service.
exec-personality-x86-64.service: Main process exited, code=exited, status=1/FAILURE
exec-personality-x86-64.service: Failed with result 'exit-code'.
...
        Exit Status: 1
src/test/test-execute.c:456:test_exec_personality: exec-personality-x86-64.service: can_unshare=yes: exit status 1, expected 0
(test-execute-root) terminated by signal ABRT.
Assertion 'r >= 0' failed at src/test/test-execute.c:1433, function prepare_ns(). Aborting.
Aborted