Revert of Reland Combined test runner. (patchset #4 id:60001 of https://codereview.chromium.org/1886753002/ )

Reason for revert:
Dr. Memory still broken: https://build.chromium.org/p/client.pdfium/builders/drm_win_xfa/builds/330/steps/pixel%20tests/logs/stdio

Original issue's description:
> Reland Combined test runner.
>
> This reverts commit 7a4374583efc0c41c826122aa26c1198c8d5cc37.
>
> Original Commit Message:
>
> This CL revives the old CL to combine the test runners [1] which was reverted
> due to failing font_size tests. I've deleted the font_size.pdf as it is not
> needed and for testing. Pixel tests are either .in or .pdf files, not both.
>
> Original description:
> This CL takes the three test runners (corpus, javascript, pixel) and combines
> the code into a single test_runner file. Each of the individual runners still
> exists and calls the test runner with their data directory.
>
> With this change, the pixel and javascript test will now run in parallel if
> multiple processors are available.
>
> 1-https://codereview.chromium.org/1430623006/
>
> Committed: https://pdfium.googlesource.com/pdfium/+/9cec54ab0a5461b3075c585f8f233dbfd06c2cbd

TBR=tsepez@chromium.org,ochang@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1890703002

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