tests: consolidate tests for true and false

* src/true.c (main): Add a comment about the possibility
of true returning EXIT_FAILURE due to write failure.
* tests/misc/false-status.sh: Fix so we're testing
the tool and not the shell builtin.  Add a case for true(1).
* tests/misc/help-version.sh: Skip /dev/full test
for true as well as false since the exit status is tested separately.
Also remove the iterations for different LC_MESSAGES, as this was only
applied for false(1).  Translations are not honored in the test dir
and so would need separate handling in any case.