Improve test_link_arg_* tests

The `test_link_arg_regex` test was accidentally passing. By adding
another link to the test html, we can see that it was selecting the
first link instead of the link that matched the supplied regex.
This new version of the test fails, but it will be fixed by #256.

Also added a case for no arguments and reduced code duplication
by changing it to a parameterized test.

tests/test_stateful_browser.py

@@ -421,18 +421,18 @@ def test_referer_submit_headers(httpbin):
     assert headers['X-Test-Header'] == 'x-test-value'
 
 
-def test_link_arg_text(httpbin):
-    browser = mechanicalsoup.StatefulBrowser()
-    browser.open_fake_page('<a href="/get">Link</a>', httpbin.url)
-    browser.follow_link(link_text='Link')
-    assert browser.get_url() == httpbin + '/get'
-
-
-def test_link_arg_regex(httpbin):
+@pytest.mark.parametrize('expected, kwargs', [
+    pytest.param('/foo', {}, id='none'),
+    pytest.param('/get', {'text': 'Link'}, id='text'),
+    pytest.param('/get', {'url_regex': 'get'}, id='regex',
+                 marks=pytest.mark.xfail),
+])
+def test_follow_link_arg(httpbin, expected, kwargs):
     browser = mechanicalsoup.StatefulBrowser()
-    browser.open_fake_page('<a href="/get">Link</a>', httpbin.url)
-    browser.follow_link(url_regex='.*')
-    assert browser.get_url() == httpbin + '/get'
+    html = '<a href="/foo">Bar</a><a href="/get">Link</a>'
+    browser.open_fake_page(html, httpbin.url)
+    browser.follow_link(**kwargs)
+    assert browser.get_url() == httpbin + expected
 
 
 def test_link_arg_multiregex(httpbin):