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

• [X] I have read the contributing guide and understand what's expected of me
• [X] I have run this code in development and it appears to resolve the stated issue
• [X] This PR includes tests, or tests are not required/relevant for this PR
• [X] 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
• [X] This PR includes type annotations for new and modified functions

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10338/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10338/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `86.04% <ø> (+0.06%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10338/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `62.22% <ø> (+0.36%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs#carryforward-flags-in-the-pull-request-comment) to find out more.

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