Sort each --test option individually, then concatenate

This preserves the order of tests on the command line, for example,
with the following hierarchy of tests:

    aaa_tests/
        00_test.rb
        01_test.rb
    zzz_tests/
        00_test.rb
        01_test.rb

And a systest command line of: `systest --tests zzz_tests --tests
aaa_test`, the previous behavior was to run in the following order:

    aaa_tests/00_test.rb
    aaa_tests/01_test.rb
    zzz_tests/00_test.rb
    zzz_tests/01_test.rb

This commit causes the harness to keep the order specified on the
command line, so the tests would be run in this order instead:

    zzz_tests/00_test.rb
    zzz_tests/01_test.rb
    aaa_tests/00_test.rb
    aaa_tests/01_test.rb