[Export Tagged PDFs] Refactor HeadlessTaggedPDFBrowserTest

The goal of this refactor was to give tests useful names and to make it possible to programmatically re-normalize test results.

The refactor follows the pattern of the "Dump Accessibility Tests": https://source.chromium.org/chromium/chromium/src/+/main:content/test/data/accessibility/readme.md?q=%22--generate-accessibility-test-expectations%22

1. Instead of hard coding expectations of the test (a json representing the structured data of the PDF file) as a string in the browsertest.cc file, the expectations are save in a file along side the test html that is used to produce the expectations.

2. In order to produce this expectation file, make sure the file exist with the correct name ("<testfile>_expected.html") and run the test with `--reset-results`.

Additionally, the HeadlessTaggedPDFDisabledBrowserTest was previously parameterized with all the tagged html files as input. This seems unnecessary (and I removed the hard coded list of files), so now it only tests one html file to confirm if tagging is disabled, a tagged PDF is not produced.

Bug: 443772810
Change-Id: I35e88a55bb6b1fba8e6b9961335dc5a006c8e5df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7017818
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Commit-Queue: Valerie Young <spectranaut@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1527742}