Fix path mangling in reftest list

The reftest list for gradients had somehow got damaged, and paths missed
a few characters. Fixed that.
Fix a few mistakes in reftest.list files
Convert old fashioned reftest lists into link tags

Replaces reftest.list files with the equivalent link tag in relevant
files, as reftest.list is considered deprecated.

Conversion done using: https://github.com/frivoal/reftest-converter
Remove upstreamed preload tests and scripts

https://github.com/w3c/web-platform-tests/commit/c8be2401626658905aa3684419aeb5dde7051a56
Remove upstreamed <track> and WebVTT tests

https://github.com/w3c/web-platform-tests/commit/ba982a2de52f9650bcb7b503c0c926f847b2c48a
Remove upstreamed resource selection tests

https://github.com/w3c/web-platform-tests/pull/326
Remove Audio constructor tests

https://github.com/w3c/web-platform-tests/pull/528
Remove redundant reflection tests

https://github.com/w3c/web-platform-tests/blob/1ae68c0331d6723e7226013f0fefe1247cacf4cf/html/dom/reflection-embedded.html
Remove media demos

There's nothing to upstream in these.
Remove tests for historical media features

https://github.com/w3c/web-platform-tests/pull/649
https://github.com/w3c/web-platform-tests/pull/653
Remove tests for the old Audio API

These tests are for the Audio interface predating HTMLMediaElement.
There doesn't seem to be anything worth salvaging.
Remove redundant interface tests

https://github.com/w3c/web-platform-tests/blob/9fe02f7cf4ea2bdc57eee8339975efbeaf489c8f/html/dom/interfaces.html
Merge pull request #14 from frivoal/reftests

Reftest updates from frivoal
Merge pull request #15 from foolip/rm-preload

Remove upstreamed preload tests and scripts
Merge pull request #16 from foolip/rm-track

Remove upstreamed <track> and WebVTT tests

There were reftest conflicts due to pull request #14 from frivoal/reftests
Merge pull request #17 from foolip/rm-resource-selection

Remove upstreamed resource selection tests
Merge pull request #18 from foolip/rm-audio-constructor

Remove Audio constructor tests
Merge pull request #19 from foolip/rm-reflection

Remove redundant reflection tests
Merge pull request #20 from foolip/rm-demos

Remove media demos
Merge pull request #21 from foolip/rm-historical

Remove tests for historical media features