Fix broken tests + Add code coverage tracking

[Ndiritu]

This PR:

• fixes failing tests as a result of refactoring done in Remove Guzzle specific config and methods from Graph request classes #4
• fixes bugs in Remove Guzzle specific config and methods from Graph request classes #4 that have been identified during testing
• increases test coverage of our GraphRequest classes

closes #18

[MIchaelMainer]

Do you have code coverage % info for before and after? This QA improvement would be useful for inclusion in quality review, as well as to put in your own review.

[Ndiritu]

Do you have code coverage % info for before and after? This QA improvement would be useful for inclusion in quality review, as well as to put in your own review.

• I've setup code coverage config using PHPUnit to this PR.
• When validating a PR, we will be able to see the coverage summary in the github actions output for now.
• Better solutions would be able to make this an action on it's own and potentially block PRs if they reduce the test coverage. PHPUnit doesn't seem to support a simple GitHub Action to do this = more work. Created a tracking item
• PHPUnit produces a bunch of metrics. Path coverage seems to be most useful one IMO

Before - (current main)

Code Coverage Report:
  2021-08-16 10:16:20

 Summary:
  Classes: 14.29% (1/7)
  Methods: 62.26% (33/53)
  Paths:   60.78% (62/102)
  Branches:   84.50% (109/129)
  Lines:   79.25% (210/265)

After - (this PR)

Code Coverage Report:
  2021-08-16 10:15:12

 Summary:
  Classes: 66.67% (6/9)
  Methods: 83.87% (52/62)
  Paths:   62.42% (93/149)
  Branches:   96.09% (172/179)
  Lines:   90.67% (243/268)

To see more detail, running coverage locally would produce a coverage/ folder (gitignored) containing HTML pages with deeper analysis of which code paths still need testing:
php -xdebug.mode=coverage ./vendor/bin/phpunit

Biggest gaps are in GraphCollectionRequest and Enum classes