🏗 Add a way to validate all HTML test fixtures, even skipped ones

[rsimha]

This PR adds a way to validate HTML fixtures that are currently skipped from checks due to existing errors.

Usage:

# Step 1a
amp validate-html-fixtures # Passes because fixtures with errors are skipped

# Step 1b
amp validate-html-fixtures --include_skipped # Fails with a huge number of errors

# Step 2
# Pick a file with validation errors from step 1b and fix it
# Optional: Add the amphtml-validator extension to VSCode for easy detection of validation errors

# Step 3
amp validate-html-fixtures --include_skipped --local_changes # Shows if the file just edited is indeed fixed

# Step 4
# Once fixed, remove the file from the allowlist in htmlFixtureGlobs in build-system/test-configs/config.js

# Repeat steps 2-4 with as many files as you'd like, and send out a PR

With this, it will become significantly easier for us to fix the huge list of validation errors in our test fixtures. See #25145 (review) and #25149 (comment) for why this is important.

Follow up to #33997
Partial fix for #25149

[jridgewell]

Is include_skipped even needed for local_changes? I'd imagine if you modified one of the fixtures, then you can take it all the way and make it valid, too.

[rsimha]

Is include_skipped even needed for local_changes? I'd imagine if you modified one of the fixtures, then you can take it all the way and make it valid, too.

Yep, --include_skipped is only needed with --local_changes if you haven't yet removed the fixture from the allowlist. In practice though, it's useful to first check if the fixture is valid before removing it from the allowlist.