tests: only activate 'unset' alias if required

tests: only activate 'unset' alias if required

This makes the test logs easier to read for most shells (which do not
actually require that alias).  This is especially important now that
'unset' is used in the new 'run_make()' function, and that function is
likely going to be used more and more in the future.

* t/ax/test-lib.sh (_am_unset, unset): Only define this function and
alias if "unset VAR" returns a non-zero exit status when VAR is already
unset.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>