cherry-pick(#1836): fix: discard trace without path (#1838)

Author: mxschmitt

Diff for: playwright/_impl/_local_utils.py

@@ -63,7 +63,7 @@ async def tracing_started(self, tracesDir: Optional[str], traceName: str) -> str
         return await self._channel.send("tracingStarted", params)
 
     async def trace_discarded(self, stacks_id: str) -> None:
-        return await self._channel.send("traceDiscarded", {"stacks_id": stacks_id})
+        return await self._channel.send("traceDiscarded", {"stacksId": stacks_id})
 
     def add_stack_to_tracing_no_reply(self, id: int, frames: List[StackFrame]) -> None:
         self._channel.send_no_reply(

Diff for: tests/async/test_tracing.py

@@ -33,6 +33,13 @@ async def test_browser_context_output_trace(
     assert Path(tmp_path / "trace.zip").exists()
 
 
+async def test_start_stop(browser: Browser):
+    context = await browser.new_context()
+    await context.tracing.start()
+    await context.tracing.stop()
+    await context.close()
+
+
 async def test_browser_context_should_not_throw_when_stopping_without_start_but_not_exporting(
     context: BrowserContext, server: Server, tmp_path: Path
 ):