It is still possible to override the executable to run during the test,
using the <tool> tag, but this patch removes the requirement that the
tag must be present for unit tests.
It also removes the possibility of human error when existing test cases
are used as the basis for new tests, as recently witnessed in 81c37124.
Reviewed-by: Daniel Stenberg
Closes#4976
The test runner script now knows if unittests can run and the unit test
setup file says it is one. I also made runtests.pl deal with no
<command> tag set, so that the description file can get even simpler.