{Core} Fix test_open_page_in_browser on WSL

[jiasli]

Description

src/azure-cli-core/azure/cli/core/tests/test_util.py::TestUtils::test_open_page_in_browser fails on WSL:

_mock_self = <MagicMock name='open' spec='function' id='139689703216128'>, args = ('http://foo', 2), kwargs = {}
self = <MagicMock name='open' spec='function' id='139689703216128'>
msg = "Expected 'open' to be called once. Called 0 times."

    def assert_called_once_with(_mock_self, *args, **kwargs):
        """assert that the mock was called exactly once and that that call was
        with the specified arguments."""
        self = _mock_self
        if not self.call_count == 1:
            msg = ("Expected '%s' to be called once. Called %s times.%s"
                   % (self._mock_name or 'mock',
                      self.call_count,
                      self._calls_repr()))
>           raise AssertionError(msg)
E           AssertionError: Expected 'open' to be called once. Called 0 times.

env38/lib/python3.8/site-packages/mock/mock.py:925: AssertionError

Now fix it!

[jiasli]

Start-Process is async, so no need to use call to wait for the result.

[yonzhan]

Core