[prtester] fix prtester no longer supporting multiple bridges

[User123698745]

This PR will fix prtester.py no longer supporting multiple bridges being changed (after #4292 by @Bockiii), because the names of the generated html files are not unique enough.

Example
Reddit and XPath bridges get changed, because of refactoring.

Current filenames would be:

• current_1.html => XPath 1 overrides Reddit 1
• current_2.html => XPath 2 overrides Reddit 2
• current_3.html => Reddit 3
• pr_1.html => XPath 1 overrides Reddit 1
• pr_2.html => XPath 2 overrides Reddit 2
• pr_3.html => Reddit 3

New filenames will be:

• Reddit_1_current.html
• Reddit_1_pr.html
• Reddit_2_current.html
• Reddit_2_pr.html
• Reddit_3_current.html
• Reddit_3_pr.html
• XPath_1_current.html
• XPath_1_pr.html
• XPath_2_current.html
• XPath_2_pr.html

Additional changes

• delete all *.html files at the start (improves local debugging for devs)
• git ignore the generated *.html files in the .github directory
• use about:blank again when no html artifact is generated (if error_messages is true or if with_upload and (not with_reduced_upload or not status_is_ok): is false)
• fix SyntaxWarning: invalid escape sequence by add r in front of regex strings

[Bockiii]

Thanks!