test-coverage: Fix recursive deleting of temporary test data

typeflag here is a value integer, not a bitmask, and in this case
FTW_F is 0, so this test will never produce anything useful...
testCoverage: Remove log
coverage: Reindent
coverage: s/Warning/warning/

This should be lower-case
coverage: Fix the location of the comment
coverage: Use a local for fetchStatistics

Rather than going half-way across the world for it...
coverage: Don't return this

It's implicit in a constructor.
coverage: Use a closure instead of this
coverage: Don't replace existing statistics with new ones

The existing code here relies on pendingFiles getting modified, which
is a bit tricky and won't work in our new prefix system. Instead, just
make ensureStatisticsFor return an existing object if it exists already.
coverage: Don't construct a new array for no good reason
coverage: Use JS_DefineFunctions
jsapi-util: Add a gjs_parse_call_args variant, that takes JS::CallArgs

JS::CallArgs is now a recommended replacement for the argc/vp pair, and
in the future, JSNative will take a JS::CallArgs directly, instead of a
argc/vp pair. For now, use the semi-deprecated "array" member to share
the same code as the old argc/vp interface.

When upstream deprecates argc/vp, we'll port everything to JS::CallArgs.

This is because I spend almost half an hour finding a bug with the old
interface (I called JS_SET_RVAL on the argv instead of vp), and they
recommended that I use JS::CallArgs instead.
coverage: Pass filenames instead of objects

This simplifies the code a bit as well and prepares us for passing
import prefixes rather than explicit filenames.
coverage: Simplify statistics creation code
coverage: Remove call_js_function_for_array_return

It gummies up the code, and won't fit our getCoveredFiles model.