Add unit tests for Graph class

[QMalcolm]

resolves #9871

Problem

We haven't been directly unit testing our Graph class, which is an integral class to the operation of dbt

Solution

Add unit tests

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.78%. Comparing base (100352d) to head (259036e).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10338      +/-   ##
==========================================
+ Coverage   88.71%   88.78%   +0.06%     
==========================================
  Files         180      180              
  Lines       22483    22490       +7     
==========================================
+ Hits        19946    19967      +21     
+ Misses       2537     2523      -14     

Flag	Coverage Δ
integration	86.04% <ø> (+0.06%)	⬆️
unit	62.22% <ø> (+0.36%)	⬆️

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