[v6.x backport] path.win32 related fixes and tests

[TimothyGu]

First commit (#14438):

commit 5180f932eaccfa380f68908b00b098205cb466ba
Author: Tobias Nießen <[email protected]>
Date:   Mon Jul 24 16:20:30 2017 +0200

    test: increase coverage for path.parse
    
    PR-URL: https://github.com/nodejs/node/pull/14438
    Reviewed-By: Refael Ackermann <[email protected]>
    Reviewed-By: James M Snell <[email protected]>
    Reviewed-By: Anna Henningsen <[email protected]>
    Reviewed-By: Luigi Pinca <[email protected]>
    Reviewed-By: Timothy Gu <[email protected]>
    Reviewed-By: Colin Ihrig <[email protected]>
    Reviewed-By: Khaidi Chu <[email protected]>

Second commit (#14440):

commit 6fa5f3575422a12d374b349e3234331ca5e734c8
Author: Timothy Gu <[email protected]>
Date:   Mon Jul 24 08:39:02 2017 +0800

    path: fix win32 volume-relative paths
    
    `path.resolve()` and `path.join()` are left alone in this commit due to
    the lack of clear semantics.
    
    PR-URL: https://github.com/nodejs/node/pull/14440
    Fixes: https://github.com/nodejs/node/issues/14405
    Reviewed-By: Refael Ackermann <[email protected]>
    Reviewed-By: Tobias Nießen <[email protected]>

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

path

[TimothyGu]

CI: https://ci.nodejs.org/job/node-test-pull-request/9626/

[tniessen]

Did you intentionally not backport 2ac0aff (part of #14438)?

[TimothyGu]

@tniessen No it was not intentional. I backported the test commit in #14438 because it caused the merge conflict in #14440 but I'll backport the other one too. Thanks for noticing!

[tniessen]

I pushed the missing commit as discussed on IRC.

CI: https://ci.nodejs.org/job/node-test-pull-request/9650/

[MylesBorins]

landed in ee98df8...d75363b