29-os/system.t fails when no perl6 is installed

[Leont]

Currently two tests are not working when working from a rakudo checkout when no rakudo is installed, because both try to find the executable in the path.

• The &chdir changes the directory processes are spawned in falsely passes, because it's negatively comparing the old cwd against empty strings.
• :cwd(...) changes the directory processes are spawned in fails visibly

I tried replacing the basename with absolute, but apparently that doesn't result in a working invokation (Unhandled exception: While looking for 'Perl6/Grammar.moarvm': no such file or directory).

I suspect the solution will be something like "if running from a checkout, execute this, otherwise execute that". Until then we might as well skip it, as it's running the wrong perl6 anyway.

[Leont]

Actually, a rather similar issue (but around Test.pm6) also occurs in t/spec/S24-testing/12-subtest-todo.t