🌱 use ValidateTestReturn for SAST tests

[spencerschrock]

What kind of change does this PR introduce?

test cleanup

• PR title follows the guidelines defined in our pull request documentation

What is the current behavior?

we only checked the result score, which produces test failures without clear actionable feedback.
I discovered this during another refactor and it was hard to figure out what the issue was.

--- FAIL: Test_SAST (0.00s)
    --- FAIL: Test_SAST/SAST_checker_should_return_failed_status_when_no_PRs_are_found (0.00s)
        ~/checks/sast_test.go:319: Expected score 0, got -1 for SAST checker should return failed status when no PRs are found
    --- FAIL: Test_SAST/Successful_SAST_checker_should_return_success_status_for_sonarcloud (0.00s)
        ~/checks/sast_test.go:319: Expected score 10, got -1 for Successful SAST checker should return success status for sonarcloud

What is the new behavior (if this is a feature change)?**

We use our test helper: ValidateTestReturn, which provides much more actionable feedback when tests fail:

--- FAIL: Test_SAST (0.00s)
    --- FAIL: Test_SAST/SAST_checker_should_return_failed_status_when_no_PRs_are_found (0.00s)
        ~/checks/sast_test.go:342: SAST checker should return failed status when no PRs are found: (-expected +actual)  utests.TestReturn{
            - 	Error:         nil,
            + 	Error:         e"internal error: probe run failure: [create finding: finding.New: invalid: ID: read '', expected 'sastToolConfigured']",
            - 	Score:         0,
            + 	Score:         -1,
            - 	NumberOfWarn:  1,
            + 	NumberOfWarn:  0,
              	NumberOfInfo:  0,
              	NumberOfDebug: 0,
              }

• Tests for the changes have been added (for bug fixes/features)

Which issue(s) this PR fixes

NONE

Special notes for your reviewer

The warn/info/debug counts were just taken as-is from the current state of main branch.

Does this PR introduce a user-facing change?

For user-facing changes, please add a concise, human-readable release note to
the release-note

(In particular, describe what changes users might need to make in their
application as a result of this pull request.)

NONE

[codecov]

Codecov Report

Merging #3872 (56ae55a) into main (9b65bde) will decrease coverage by 6.70%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3872      +/-   ##
==========================================
- Coverage   75.47%   68.77%   -6.70%     
==========================================
  Files         235      235              
  Lines       15793    15793              
==========================================
- Hits        11919    10861    -1058     
- Misses       3133     4261    +1128     
+ Partials      741      671      -70