Make wheel header installation test more explicit

[chrahunt]

We have one test that headers in a wheel are handled correctly. Previously, this test was just checking that the .dist-info folder was getting created. Now we check that the header file is actually installed. I also took the opportunity to replace a custom-created wheel with our make_wheel helper.

For reference, the contents of the removed tests/data/packages/headers.dist-0.1-py2.py3-none-any.whl:

-------------------------------- headersdist.py --------------------------------

-------------------- headers.dist-0.1.data/headers/header.h --------------------

------------------- headers.dist-0.1.dist-info/metadata.json -------------------
{"license": "UNKNOWN", "name": "headers.dist", "metadata_version": "2.0", "author": "UNKNOWN", "home_page": "UNKNOWN", "summary": "A distribution with headers", "author_email": "UNKNOWN", "version": "0.1", "platform": "UNKNOWN"}
----------------------- headers.dist-0.1.dist-info/WHEEL -----------------------
Wheel-Version: 1.0
Generator: bdist_wheel
Root-Is-Purelib: true
Tag: py2-none-any
Tag: py3-none-any


--------------------- headers.dist-0.1.dist-info/METADATA ----------------------
Metadata-Version: 2.0
Name: headers.dist
Version: 0.1
Summary: A distribution with headers
Home-page: UNKNOWN
Author: UNKNOWN
Author-email: UNKNOWN
License: UNKNOWN
Platform: UNKNOWN

UNKNOWN



---------------------- headers.dist-0.1.dist-info/RECORD -----------------------

headersdist.py and metadata.json did not seem relevant to the current test, so they have been omitted in the wheel we now construct at runtime.

Progresses #6030.

[chrahunt]

Thank you for reviewing!