Add unit tests for twine.utils

[deveshks]

Towards #7 .

Add more unit tests to twine.utils to bring the coverage up to 99%

py run-test: commands[1] | coverage report -m
Name                         Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------
twine/__init__.py               14      1      2      1    88%   29->32, 32
twine/_installed.py             40      3     18      7    83%   19->56, 21->22, 22, 26->56, 45->56, 46->51, 48->52, 51, 52->45, 56
twine/auth.py                   55      0      4      1    98%   55->62
twine/cli.py                    25      0      4      0   100%
twine/commands/__init__.py      21      0     11      0   100%
twine/commands/check.py         72      0     24      1    99%   90->97
twine/commands/register.py      23      8      4      2    63%   28->29, 29, 38->43, 43, 47-62
twine/commands/upload.py        55      2     22      2    95%   77->78, 78, 79->80, 80
twine/exceptions.py             27      0      0      0   100%
twine/package.py               128      8     35      8    90%   83->88, 88, 101->102, 102-103, 106->107, 107, 163->164, 164, 167, 236->exit, 240->242, 242, 245->exit, 249->251, 251
twine/repository.py            120     15     38      6    85%   121-136, 183->201, 188->189, 189-190, 201, 208->209, 209, 214->217, 217->227, 229->230, 230-231, 251
twine/settings.py               68      0      4      0   100%
twine/utils.py                  98      0     40      1    99%   172->177
twine/wheel.py                  46      3     16      5    87%   56->57, 57, 59->67, 67, 70->76, 73->70, 76->78, 78
twine/wininst.py                37     26     19      0    20%   14-16, 20-24, 27-56
------------------------------------------------------------------------
TOTAL                          829     66    241     34    89%

[bhrutledge]

Thanks for this! Just a quick note as you continue your efforts.