Re-add tests/ directory

pyproject.toml

@@ -305,13 +305,15 @@ omit = [
     "*/trio/_tests/check_type_completeness.py",
     # Script to generate a CFFI interface for the Windows kernel
     "*/trio/_tools/windows_ffi_build.py",
+    # pytest plugin (I'm not sure why this doesn't work)
+    "tests/_trio_check_attrs_aliases.py",
 ]
 # The test suite spawns subprocesses to test some stuff, so make sure
 # this doesn't corrupt the coverage files
 parallel = true
 plugins = []
 relative_files = true
-source = ["trio"]
+source = ["trio", "tests/"]
 
 [tool.coverage.report]
 precision = 1

src/trio/_core/_tests/test_thread_cache.py

@@ -220,23 +220,8 @@ def foo() -> None:
     if child_pid != 0:
         # if this test fails, this will hang, triggering a timeout.
         os.waitpid(child_pid, 0)
-    else:
-        # this is necessary because os._exit doesn't unwind the stack,
-        # so coverage doesn't get to automatically stop and save
-        # coverage information.
-        try:
-            import coverage
-
-            cov = coverage.Coverage.current()
-            # the following pragmas are necessary because if coverage:
-            #  - isn't running, then it can't record the branch not
-            #    taken
-            #  - isn't installed, then it can't record the ImportError
-
-            if cov:  # pragma: no branch
-                cov.stop()
-                cov.save()
-        except ImportError:  # pragma: no cover
-            pass
-
-        os._exit(0)  # pragma: no cover  # coverage was stopped above.
+    else:  # pragma: no cover  # coverage is shut down by os._exit(0)
+        # we would *want* to allow coverage to take a snapshot here. check
+        # git glame for how to do that. however, that times out for some
+        # reason when having `tests/` in `source` for coverage.py.
+        os._exit(0)

src/trio/_core/_thread_cache.py

@@ -303,7 +303,9 @@ def start_thread_soon(
     THREAD_CACHE.start_thread_soon(fn, deliver, name)
 
 
-def clear_worker_threads() -> None:
+def clear_worker_threads() -> (
+    None
+):  # pragma: no cover # see test_clear_thread_cache_after_fork
     # This is OK because the child process does not actually have any
     # worker threads. Additionally, while WorkerThread keeps a strong
     # reference and so would get affected, the only place those are