Adding markdown test cases

[lmpham1]

I have added the following test cases for the MDUtils class:

• test bold markdown
• test header markdown
• test hr markdown
• test code block markdown

The following tests did not pass, I have commented out:

• bold markdown in the middle of a string, i.e.: Testing *bold* markdown, is not detected
• unclosed code block markdown is detected as valid code block, but it shouldn't
• invalid horizontal rule, i.e. ----invalid-hr is detected as valid hr