process00{1,2} tests: just assume ls is in the PATH on non-Windows systems

[alpmestan]

This should work on all the systems I've ever laid my hands on (not many, admittedly) and of particular interest to me and several other GHC/core libraries contributors who run GHC's testsuite, with this patch those tests now pass on NixOS.

$ TEST="process001 process002" ./validate --fast --testsuite-only
[...]
==== STAGE 2 TESTS ==== 

SUMMARY for test run started at Tue Jun 12 15:51:02 2018 CEST
 0:00:01 spent to go through
       2 total tests, which gave rise to
      14 test cases, of which
      12 were skipped

       0 had missing libraries
       2 expected passes
       0 expected failures

       0 caused framework failures
       0 caused framework warnings
       0 unexpected passes
       0 unexpected failures
       0 unexpected stat failures

There, ls is not under /bin but somewhere in the nix store and brought into the PATH through symlinks in a "current environment" directory which itself is in the PATH.

[snoyberg]

@bgamari Since this affects the GHC test suite and not the process test suite, would you mind giving it a review? I don't see any issues myself, but I'm not very familiar with GHC's test suite setup.

@Mistuke The changes here touch on Windows-specific code, but seem to leave the Windows code path identical to what it was before, so I don't believe it should affect Win32 at all.

[int-index]

@snoyberg Ben has expressed his approval of this change here: alpmestan/ghc.nix#10 (comment)

[snoyberg]

Thanks @int-index