Bug 1790542 - [devtools] Convert browser_source_map-reload.js test from using...

Bug 1790542 - [devtools] Convert browser_source_map-reload.js test from using .sjs files to createTestHTTPServer r=ochameau,devtools-reviewers

.sjs files depend on redirects to load static files.
In D168648 we block redirects on sourceMappingUrl, so this browser_source_map-reload.js
fails because we use .sjs to serve the content.

This patch refactors to no longer use .sjs files

Differential Revision: https://phabricator.services.mozilla.com/D169739