Fix IE11 incompatibilities

[Telroshan]

Description

Fixes some IE11 incompatibilities

• default parameter value in function makeTagRegEx(tag, global = false), not supported by IE11
• string literals in that same function, not supported by IE11 either
• the test utility functions call htmx.processNode on text nodes (i.e. not actual Elements). This fails gracefully on modern browsers but not so gracefully on IE11. Added 2 checks in hx-on processing functions, to avoid calling undefined references (a text node doesn't have .attributes neither getElementsByTagName)
• some tests cannot work on IE11 (unsupported features) but were simply not skipped, fixed that

Corresponding issue:

Testing

See #1687 with the procedure to run tests on IE11

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded