wpt: Make legacy ServoWdspecBrowser work as intended (servo#40296)

servo#40289 (comment) intend
to process `headless` param properly.

For either `servo` or `servodriver` binary, the expected behaviour is:
- When given one test, it should depends on cmd arg `--headless`
- With multiple tests, we always default to `--headless`

The subtle point is: `ServoWdspecBrowser` didn't process `headless`
param at all and always default to headless. Also, `executor_kwargs`
does not work for it but only `browser_kwargs` works as it does not use
`ServoExecutor`.

Testing: Manually tested with legacy driver, running in single/multiple
tests, with/without headless param, with wdspec or none-wdspec test.
Fixes: servo#40289 (comment)

---------

Signed-off-by: Euclid Ye <[email protected]>

Author: yezhizhen

tests/wpt/meta/MANIFEST.json

@@ -533229,7 +533229,7 @@
         ]
        },
        "servo.py": [
-        "b8cfd54ef61e5cdde9bef4e3673e508759adc566",
+        "7a95b08e7ad9ef88be26dfdaf4547b198a0a1439",
         []
        ],
        "servodriver.py": [

tests/wpt/tests/tools/wptrunner/wptrunner/browsers/servo.py

@@ -42,6 +42,7 @@ def browser_kwargs(logger, test_type, run_info_data, config, subsuite, **kwargs)
         "binary": kwargs["binary"],
         "debug_info": kwargs["debug_info"],
         "binary_args": kwargs["binary_args"] + subsuite.config.get("binary_args", []),
+        "headless": kwargs["headless"],
         "user_stylesheets": kwargs.get("user_stylesheets"),
         "ca_certificate_path": config.ssl_config["ca_cert_path"],
     }
@@ -96,6 +97,7 @@ class ServoWdspecBrowser(WebDriverBrowser):
     # TODO: could share an implemenation with servodriver.py, perhaps
     def __init__(self, logger, binary="servo", webdriver_binary="servo",
                  binary_args=None, webdriver_args=None, env=None, port=None,
+                 headless=None,
                  **kwargs):
 
         env = os.environ.copy() if env is None else env
@@ -109,6 +111,8 @@ def __init__(self, logger, binary="servo", webdriver_binary="servo",
                          env=env,
                          **kwargs)
         self.binary_args = binary_args
+        self.headless = ["--headless"] if headless else None
+
 
     def make_command(self):
         command = [self.binary,
@@ -117,10 +121,11 @@ def make_command(self):
                    # See https://github.com/servo/servo/issues/30080.
                    # For some reason rustls does not like the certificate generated by the WPT tooling.
                    "--ignore-certificate-errors",
-                   "--headless",
                    "--window-size",
                    "800x600",
                    "about:blank"] + self.webdriver_args
         if self.binary_args:
             command += self.binary_args
+        if self.headless:
+            command += self.headless
         return command