SpiderMonkey now provides its own coverage data conveniently already
formatted as LCOV output. It does have a few quirks and so requires a bit
of adjustment in the tests, but it allows us to drop a few thousand lines
of custom code.

Differences between the old LCOV data and the new:

 - There is always a function named "top-level" in each file.
 - Anything on line 1 seems to be ignored.
 - Branches can appear in different order.
 - There's no difference between not-taken and not-executed branches.
   They always appear as not-executed.

https://bugzilla.gnome.org/show_bug.cgi?id=788166