test: Misc Windows unit test fixes

Fixes #5071, #5073.

* Normalize capitalization of drive letter
* Fix `exit()` typo in failure path
* Ignore symlink tests (Windows) if not elevated

The `test_relative_input_cwd()` test was failing on Windows when
`skipSymlinks` was `true`. So we won't run it if `skipSymlinks` is
`true`.

When it failed, the unhandled error caused Node to die before
having a chance to clean up, which resulted in two files missing
in subsequent unit tests:

 * `test/fixtures/nested-index/one/hello.js`
 * `test/fixtures/nested-index/one/index.js`

We should probably find a way to isolate this test from the other
test (`simple/test-module-loading`) that was failing when this test
poluted the disk state.