Increase test coverage to 100%

[adamchainz]

Currently at 96% - the report can be seen and downloaded in HTML format from any run on master.

Here's a version at time of creating issue:

 Name                                                                 Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------------------------------------------------------
src/whitenoise/__init__.py                                               3      0      0      0   100%
src/whitenoise/base.py                                                 177      5     84      9    95%   142, 155->152, 164, 171->179, 188, 211->213, 235->exit, 257, 261
src/whitenoise/compress.py                                              84      0     34      1    99%   169->168
src/whitenoise/media_types.py                                           17      0      6      0   100%
src/whitenoise/middleware.py                                           108      4     46      5    94%   88->91, 89->91, 92, 126->130, 132, 170-171
src/whitenoise/responders.py                                           202      5     92      9    95%   92-96, 109->111, 129, 134, 145->147, 170->176, 176->181, 229->exit, 277->280, 285
src/whitenoise/runserver_nostatic/__init__.py                            0      0      0      0   100%
src/whitenoise/runserver_nostatic/management/__init__.py                 0      0      0      0   100%
src/whitenoise/runserver_nostatic/management/commands/__init__.py        0      0      0      0   100%
src/whitenoise/runserver_nostatic/management/commands/runserver.py      26      3     12      4    82%   20->exit, 24-25, 37->34, 39, 48->exit
src/whitenoise/storage.py                                              120      3     58     10    93%   29->31, 36->exit, 49, 92, 96->109, 100->109, 135->137, 149->151, 167->169, 187
src/whitenoise/string_utils.py                                          12      1      4      1    88%   6
tests/__init__.py                                                        0      0      0      0   100%
tests/conftest.py                                                        9      0      0      0   100%
tests/django_settings.py                                                14      0      0      0   100%
tests/django_urls.py                                                     2      0      0      0   100%
tests/test_compress.py                                                  57      0      6      0   100%
tests/test_django_whitenoise.py                                        139      0     16      0   100%
tests/test_media_types.py                                               18      0      0      0   100%
tests/test_runserver_nostatic.py                                        10      0      0      0   100%
tests/test_storage.py                                                   63      0     16      0   100%
tests/test_string_utils.py                                              26     13      4      0    57%   8, 11-12, 15-18, 23, 26, 29, 32, 35, 38
tests/test_whitenoise.py                                               241      0     26      0   100%
tests/utils.py                                                          44      0     12      0   100%
----------------------------------------------------------------------------------------------------------------
TOTAL                                                                 1372     34    416     39    96%

After getting it to 100% we can enforce it on CI.