Fixes and completes install_addon() tests on firefox (#1609)

test_install_addon_directory() was being called with an xpi file
fixture instead of a directory fixture.

I fixed this by making a new fixture for
test_install_addon_directory() that points to the test extension
directory. See details here below.

I also added an additional test and fixture so that the tests
take into account whether a path to an extension directory ends
with a trailing slash. See details here below.

This required renaming the existing addon_path fixture to avoid
confusion among the three related fixtures:

- addon_path_xpi

  renamed fixture gives a path to the xpi file (was addon_path)

  updated test_install_add() and test_unistall_addon()

- addon_path_dir

  new fixture that gives a path to the extension directory,
  ending without a '/'

  updated test_install_unsigned_addon_directory() to use it

- addon_path_dir_slash (new fixture, and test)

  new fixture that gives a path to the extension directory,
  ending with a '/'

  added new test: test_install_unsigned_addon_directory_slash()

Note that I don't have the appropriate environment to run these tests
before pushing them.

[deploy site] 14a74c9

Author: selenium-ci

documentation/_print/index.html

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