Add code coverage for injecting modules on command line

[shahzebsiddiqui]

We are not testing buildtest build --modules and buildtest build --unload-modules in the test coverage. This can be found in https://app.codecov.io/gh/buildtesters/buildtest/blob/devel/buildtest/builders/base.py under following section.

In addition we might also want to test for module purge buildtest build --module-purge

[shahzebsiddiqui]

@kingxleyy i will be assigning this task to you

[shahzebsiddiqui]

HINT

• Add your test method https://github.com/buildtesters/buildtest/blob/devel/tests/cli/test_build.py and invoke the BuildTest class. It has method arguments to pass the modules.
• Run the regression test see this docs page for guidance. I would run all the test together. Then view the coverage results locally to see if you have coverage. Alternately you can view changes in codecov if you make PR the changes will automatically show up