Add option to limit concurrency of tests

The main purpose of this is avoiding timeouts in CI.

Passing `--max-concurrency=n` to `test_driver.py` will result in at most `n`
tests running concurrently, where `n` is a positive integer.
Not specifying the argument preserves the old behavior of running all tests
concurrently without a limit.

tests/test_driver.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 import argparse
 import asyncio
+import contextlib
 from dataclasses import dataclass
 from datetime import datetime
 import os
@@ -115,10 +116,20 @@ async def main():
     argparser = argparse.ArgumentParser(
         description="test_driver: Run fish's test suite"
     )
+    argparser.add_argument(
+        "--max-concurrency",
+        type=int,
+        help="Maximum number of tests to run concurrently. The default is to run all tests concurrently.",
+    )
     argparser.add_argument("fish", nargs=1, help="Fish to test")
     argparser.add_argument("file", nargs="*", help="Tests to run")
     args = argparser.parse_args()
 
+    max_concurrency = args.max_concurrency
+    if max_concurrency is not None and max_concurrency < 1:
+        print("--max-concurrency must be at least 1")
+        sys.exit(1)
+
     fishdir = Path(args.fish[0]).absolute()
     if not fishdir.is_dir():
         fishdir = fishdir.parent
@@ -176,10 +187,19 @@ async def main():
             tmp_root / "fish_test_helper",
         )
 
-        tasks = [
-            run_test(tmp_root, f, arg, script_path, def_subs, lconfig, fishdir)
-            for f, arg in files
-        ]
+        semaphore = (
+            asyncio.Semaphore(max_concurrency)
+            if max_concurrency
+            else contextlib.nullcontext()
+        )
+
+        async def run(f, arg):
+            async with semaphore:
+                return await run_test(
+                    tmp_root, f, arg, script_path, def_subs, lconfig, fishdir
+                )
+
+        tasks = [run(f, arg) for f, arg in files]
         for task in asyncio.as_completed(tasks):
             result = await task
             # TODO(python>3.8): use match statement