Expand "ls --color" tests to also use the dircolors defaults.

Expand "ls --color" tests to also use the dircolors defaults.

* tests/misc/ls-misc: Adjust this test so each is run twice.  First, as usual,
and the second time with LS_COLORS set using the default settings
produced by running dircolors.
* tests/Coreutils.pm: See below.

This required some changes:
  - save and restore $ENV{LS_COLORS} for each individual test
  that requires a specific value.
  - Since this is the first test to call the run_tests command more
  than once, it has exposed that that function erroneously modifies
  the \@Tests array.  Fix that in tests/Coreutils.pm.