test: make tests cwd-independent

[vsemozhetbyt]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

Editing and checking various tests, I've sometimes stumbled on cases when tests failed depending on cwd. So I've run all the tests with this simple script to find out cwd-dependent tests. The script runs all tests from repo root (default test runner tools/test.py behavior) as well as from these ones: root/.., root/test, root/test/<test subdirectory>. If a test fails in all cases or in no cases, it is loosely considered cwd-independent.

So I've found out these 3 cwd-dependent tests:

1. parallel/test-process-chdir.js can only be run outside of its directory (it makes too optimistic assumption that process.cwd() always !== __dirname after test start).
2. doctool/test-doctool-html.js can only be run from the repo root.
3. parallel/test-cli-eval.js can only be run from the repo root. It tests some murky old regression bug in the critical fragment, so I've tried to be extra careful there.

After this PR, these tests can be launched from any cwd.

[vsemozhetbyt]

CI: https://ci.nodejs.org/job/node-test-pull-request/7830/

[Trott]

LGTM if CI is green.

It might be over-engineering, but perhaps a way to guard against this coming up with future tests would be to to have the common module do a chdir() to the expected directory. I'm not actually sure how I feel about that...

[addaleax]

Landed in f1d593c