manually postprocess tests.js source map:

• drop huge sourcesContent, we just need the original source
  present when in-browser debugging…
• fix up relative paths to system components, which are in
  absolute /usr or so, by perlre(1), for repro-builds #1029670
• as a side effect, the bogus file key browserify generated also
  vanishes (now done for all browserify output)

(cherry picked from commit a575be0c)