tests: allow test options to be overridden by env

Specially for automated builds the user may want to have some
fine granulated influence like

export TS_OPT_libmount_verbose="yes"
export TS_OPT_ipcs_fake="yes"
export TS_OPT_ipcs_limits2_fake="no"
if <big endian> then
  export TS_OPT_hexdump_fake="yes"
fi
make check

Even for interactive developers this could be useful for example to
debug just one particuar test while having a regular run with stress:

TS_OPT_script_verbose="yes" \
TS_OPT_script_memcheck="yes" \
../tests/run.sh --parallel=256 --srcdir=.. --builddir=. --nonroot

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: add env option "skip"

This does just the same like "fake" but writes a more clear message
to the build log.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: add exit case "KNOWN FAILED"

Now we can mark tests as known to be broken without bothering
users with exit failure. In the build log this "KNOWN FAILED"
may be interpreted as TODO ;)

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>