4498. [test] Simplify prerequisite checks in system tests.
[RT #43516]
(cherry picked from commit 5480a74b70)
(cherry picked from commit 45fd95544c)
(cherry picked from commit 6e09e18400)
3839. [test] Use only posix-compatible shell in system tests.
[RT #35625]
(cherry picked from commit 60988462e5)
(cherry picked from commit 812cf443bb)
to be tested was not configured in at build time, it can now return 255,
and run.sh will print "R:SKIPPED" instead of "R:UNTESTED". Robie will be
able to flag this as green rather than yellow.