Add metadata.xml tests.

[allenh1]

This adds tests for metadata_xml.py. Currently, we just check that the xml parses.

[allenh1]

Ok, tests added. Current coverage is below.

allenh1@hunter-laptop ~/work/superflore $ coverage run --source=superflore -m "nose" && coverage report -m 
..............................
----------------------------------------------------------------------
Ran 30 tests in 1.867s

OK
Name                                               Stmts   Miss  Cover   Missing
--------------------------------------------------------------------------------
superflore/CacheManager.py                            20     13    35%   23-24, 28-33, 37-41
superflore/TempfileManager.py                         34     23    32%   27-28, 31-41, 44-57
superflore/__init__.py                                10      4    60%   5-8
superflore/docker.py                                  32     12    62%   40, 43-59
superflore/exceptions.py                              12      3    75%   18, 28, 33
superflore/generate_installers.py                     57     48    16%   32-86
superflore/generators/__init__.py                      0      0   100%
superflore/generators/bitbake/__init__.py              3      1    67%   3
superflore/generators/bitbake/gen_packages.py        115     92    20%   40-103, 110-166, 174-182, 188
superflore/generators/bitbake/ros_meta.py             27     18    33%   24-27, 30-35, 38-51, 54-55
superflore/generators/bitbake/run.py                 113     94    17%   39-44, 51-199
superflore/generators/bitbake/yocto_recipe.py        112     92    18%   43-64, 67, 70-74, 77-89, 92-96, 99-101, 104-105, 113-116, 125-177
superflore/generators/ebuild/__init__.py               3      1    67%   3
superflore/generators/ebuild/ebuild.py               135      7    95%   125-127, 131-136
superflore/generators/ebuild/gen_packages.py         151    125    17%   45-113, 118-150, 155-211, 216-233, 236, 239
superflore/generators/ebuild/metadata_xml.py          33      0   100%
superflore/generators/ebuild/overlay_instance.py      37     25    32%   27-32, 35-45, 48-64, 67-68
superflore/generators/ebuild/run.py                  157    137    13%   40-43, 47-52, 59-250
superflore/repo_instance.py                           49     34    31%   26-35, 38-46, 49-57, 63, 69, 75, 81, 84-91
superflore/rosdep_support.py                          34      2    94%   83-84
superflore/utils.py                                  126     53    58%   28, 32, 36, 40, 44-50, 54-59, 66, 104, 108, 114, 119, 121-124, 126, 132, 138-139, 144, 148-149, 156-177
--------------------------------------------------------------------------------
TOTAL                                               1260    784    38%