Stabilize test checking badge generation and skip windows incompatible tests

[kkapka]

Environment:

• Windows 10 Pro
• Python 3.8.3

Problem:

• Several unit tests were failing after making the clean setup.
• The first problem was, that the badge generated by the script had attributes in a different order than badge from the existing one. The comparison was done by comparing their strings
• The second issue is that the method call urllib.parse.urlparse(url) in pybadges._embed_image parse the windows path with schema (eg for path C:\Users\kkapka\something, urlparse method identifies C as a schema which causes an error, that this schema is not supported)

Solution:

• compare two badges as text with xmldiff library. That lib solves the problem with attributes order
• skip windows incompatible tests

Results:

• All tests should pass both on Linux and Windows