tests: allow test options to be overridden by env

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>