qunit: add support for precise coverage reporting

You can now do something like this:

  $ pytest -k test_browser --js-cov-files='*/fsinfo.ts'

and get results like this:

  pkg/lib/cockpit/fsinfo.ts:33:49-33:63 json_merge
                                                         ▼ from here
      33         const updated = is_json_dict(current) ? { ...current } : { };
                                                                      ▲ to here

  pkg/lib/cockpit/fsinfo.ts:36:33-38:14 json_merge
                                         ▼ from here
      36             if (value === null) {
      37                 delete updated[key];
      38             } else {
                     ▲ to here

Since we also use the same approach for collecting data for integration
tests (Profiler.takePreciseCoverage), we could easily add support there
as well.

HACKING.md

@@ -154,6 +154,22 @@ run individual tests via `pytest -k`, like so:
 
     pytest -k test-fsinfo.html
 
+You can see JavaScript code coverage information for QUnit tests.  For a
+summary table:
+
+    pytest -k test_browser --js-cov
+
+And for detailed output on uncovered sections in a specific file, something
+like:
+
+    pytest -k test-fsinfo.html --js-cov-files='*/fsinfo.ts'
+
+Coverage information is gathered into the pytest tmpdir, regardless of which
+coverage-related commandline flags are given, so it's also possible to drill
+down after the fact — without re-running tests — using something like:
+
+    test/common/js_coverage.py -m '*/fsinfo.ts' /tmp/pytest-of-*/pytest-current/js-coverage/*
+
 There are also static code and syntax checks which you should run often:
 
     test/static-code

pyproject.toml

@@ -65,6 +65,7 @@ module = [
 
     # test/common
     'testlib',
+    'js_coverage',
     'webdriver_bidi',
 ]
 
@@ -155,6 +156,7 @@ paths = [
 ]
 ignore_names = [
    "do_*",
+   "pytest_*",
    "test[A-Z0-9]*",
 ]
 ignore_decorators = [