Convert simpletest to BrowserTests

I can try later to see if your patch works for me locally.

Once I had a test failure on Drupal.org that I couldn't reproduce locally. After very many patch attempts I figured out it was a timing issue, where two things happened faster on Drupal.org, such that locally they happened with a different timestamp (a different second), but on Drupal.org they happened with the same timestamp (in the same second).

I doubt it is the same issue here but thought I'd mention it just in case. Maybe different browser versions with this issue? It looked like these tests are using some specific version of Chrome.

Thanks for doing this tests conversion, it is a big contribution.

Thanks, this is great.

#25 seems to combine (1) converting away from SimpleTest, and (2) making formatting improvements (e.g. converting to single quotes for consistency). I think it might be clearer to break into separate patches. I can split it later.

The History constraint behavior ought to be changed per #2867320: Password Policy History module wrong behaviour, in my opinion. The test logic will probably need to be adjusted for that, so I'm not going to review it too closely for now.

Committed a change to convert all double quotes in string literals of tests to single quotes.

Updated the patch accordingly, and removed a couple formatting improvements that were unrelated to this conversion. Doing so makes it easier to see in the patch/commit precisely which changes were made to convert to PHPUnit tests. (I'm apparently not the only one who thinks this is a good idea--see #7 under "Extended instructions for converting a DrupalWebTestCase test" at https://www.drupal.org/docs/8/testing/converting-d7-simpletests-to-drupal-8)

Gendered language on password_policy_history/tests/src/FunctionalJavascript/PasswordHistoryTest.php:128.

- Removed gendered language.
- Corrected incorrect article "an" to "a".
- Changed drupalget() to drupalGet() for consistency. (Unsure how that's working in all lowercase.)

Thanks Jereon.