[epic] List of custom assertions / actions

[mattgrill]

Thanks @dawehner

Things we should validate:

• What makes sense in JS
• What already exists in nightwatch and we should not duplicate. Having some form of "BC" layer would help to port the existing tests though
• Figure out whether we want all the negations etc.

Custom commands

Coming from core/tests/Drupal/Tests/BrowserTestBase.php.

• login
• get the current URL
• Click a link
• Do an select
• Handle batch pages
• Provide a nice way to fill a form with many values instead of setting values individually?

Custom asserts

Coming from vendor/behat/mink/src/WebAssert.php as well as core/tests/Drupal/Tests/WebAssert.php.

• buttonExists
• buttonNotExists
• selectExists
• optionExists
• optionNotExists
• titleEquals
• linkExists
• linkExistsExact
• linkNotExists
• linkNotExistsExact
• linkByHrefExists
• linkByHrefNotExists
• assertNoEscaped
• assertEscaped
• fieldDisabled
• hiddenFieldExists
• hiddenFieldNotExists
• hiddenFieldValueEquals
• hiddenFieldValueNotEquals
• addressEquals
• addressNotEquals
• addressMatches
• cookieEquals
• cookieExists
• pageTextContains
• pageTextNotContains
• pageTextMatches
• pageTextNotMatches
• responseContains
• responseNotContains
• responseMatches
• responseNotMatches
• elementsCount
• elementExists
• elementNotExists
• elementTextContains
• elementTextNotContains
• elementContains
• elementNotContains
• elementAttributeExists
• elementAttributeContains
• elementAttributeNotContains
• fieldExists
• fieldNotExists
• fieldValueEquals
• fieldValueNotEquals
• checkboxChecked
• checkboxNotChecked