libstd Process tests depend on msys programs on windows

[retep998]

If you don't have some sort of msys in your PATH, then the libstd process tests will fail because they attempt to spawn programs that don't exist on Windows by default.

failures:

---- process::tests::test_finish_once stdout ----
        thread 'process::tests::test_finish_once' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified." } }', src\libcore\result.rs:845

---- process::tests::test_finish_twice stdout ----
        thread 'process::tests::test_finish_twice' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified." } }', src\libcore\result.rs:845

---- process::tests::test_process_output_error stdout ----
        thread 'process::tests::test_process_output_error' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified." } }', src\libcore\result.rs:845

---- process::tests::test_process_output_output stdout ----
        thread 'process::tests::test_process_output_output' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified." } }', src\libcore\result.rs:845

---- process::tests::test_process_status stdout ----
        thread 'process::tests::test_process_status' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified." } }', src\libcore\result.rs:845

---- process::tests::test_wait_with_output_once stdout ----
        thread 'process::tests::test_wait_with_output_once' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified." } }', src\libcore\result.rs:845


failures:
    process::tests::exit_reported_right
    process::tests::smoke
    process::tests::stdout_works
    process::tests::test_finish_once
    process::tests::test_finish_twice
    process::tests::test_process_output_error
    process::tests::test_process_output_output
    process::tests::test_process_status
    process::tests::test_wait_with_output_once

[brson]

It's probably best for the test suite to run it's own binaries for testing processes, but in the short term we can just make their names unix/windows-specific.

[brson]

The task here should be simple. Identify equivalent windows system exes to call for the failing tests and use cfgs to enable them on windows.

[abhijeetbhagat]

i can work on this.

[petrochenkov]

All run-make tests also fail when run without msys (they are not skipped on MSVC, at least in rustbuild).
In theory they could use MSVC's nmake instead of make, they should be compatible for very simple makefiles.

[alexcrichton]

@petrochenkov oh my plan with run-make was to migrate all the tests to Rust actually one day rather than prolonging the life of make. I feel like it'd be a never-ending series of painful PRs to maintain compatibility with both make and nmake, so we could solve both by making a rustbuild-like transition, just rewriting all the tests in Rust.