test: clear the last error in the parse errors worker script (#2799)

#2790 stopped resetting
`error_get_last()` between worker requests, so
`TestWorkerSurvivesRequestParseErrors` fails on main.

The worker script now calls `error_clear_last()` itself, as the worker
docs recommend.

Author: dunglas

testdata/request-parse-errors.php

@@ -16,6 +16,8 @@
     if ($lastError && $lastError['type'] === E_WARNING) {
         http_response_code(400);
         echo $lastError['message'];
+        // The last error persists between requests like userland state
+        error_clear_last();
     }
 
     ++$requests;

worker_test.go

@@ -68,7 +68,7 @@ func TestWorkerSurvivesRequestParseErrors(t *testing.T) {
 		assert.Contains(t, body2, "requests:2")
 		assert.Contains(t, body2, "exceeds the limit")
 
-		// clean request: recorded errors must be reset between requests
+		// clean request: the script cleared the previous error itself
 		body3, resp3 := testGet("http://example.com/request-parse-errors.php", handler, t)
 		assert.Equal(t, http.StatusOK, resp3.StatusCode)
 		assert.Contains(t, body3, "requests:3")