test: Make sure we run lcov from the meson source directory

In ac75c51927, we accidentally changed
the working directory that the tools executed in the wrapper script
are invoked in. This broke our invocations of lcov. Let's explicitly
run those in the meson source directory again to fix the coverage
workflow.

test/integration-test-wrapper.py

@@ -326,6 +326,7 @@ def process_coverage(args: argparse.Namespace, summary: Summary, name: str, jour
                     '--quiet',
                 ],
                 check=True,
+                cwd=os.fspath(args.meson_source_dir),
             )  # fmt: skip
 
             subprocess.run(
@@ -338,6 +339,7 @@ def process_coverage(args: argparse.Namespace, summary: Summary, name: str, jour
                     '--quiet',
                 ],
                 check=True,
+                cwd=os.fspath(args.meson_source_dir),
             )  # fmt: skip
 
             Path(f'{output}.new').unlink()