feat: Refactor and add tests for load-mock

[markstory]

Refactor load-mocks into a set of smaller functions, and have some high level tests to ensure that load-mocks will run. These tests aren't exhaustive but they should help us catch and prevent regressions in load-mocks in the future.

[corps]

The diff is hard to follow, but that's understandable, it has no tests and is a dev tool. So I'm taking your word that this works locally, I do think the approach is good 👍

[codecov]

Codecov Report

Merging #53302 (f3ccc63) into master (0b666f0) will increase coverage by 0.02%.
The diff coverage is 66.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #53302      +/-   ##
==========================================
+ Coverage   79.51%   79.54%   +0.02%     
==========================================
  Files        4939     4941       +2     
  Lines      208029   208431     +402     
  Branches    35474    35521      +47     
==========================================
+ Hits       165422   165787     +365     
- Misses      37569    37595      +26     
- Partials     5038     5049      +11     

Impacted Files	Coverage Δ
static/app/components/smartSearchBar/utils.tsx	84.56% <ø> (ø)
static/app/utils/analytics/integrations/index.ts	100.00% <ø> (ø)
...tatic/app/views/releases/detail/overview/index.tsx	1.90% <ø> (ø)
.../detail/overview/sidebar/projectReleaseDetails.tsx	33.33% <ø> (ø)
...zationGeneralSettings/organizationSettingsForm.tsx	90.90% <ø> (ø)
...onIntegrations/abstractIntegrationDetailedView.tsx	75.22% <ø> (ø)
...ganizationIntegrations/integrationDetailedView.tsx	66.27% <ø> (ø)
src/sentry/utils/mockdata/core.py	66.47% <66.47%> (ø)
src/sentry/middleware/auth.py	92.20% <100.00%> (-0.10%)	⬇️
src/sentry/utils/mockdata/__init__.py	100.00% <100.00%> (ø)

... and 31 files with indirect coverage changes

[markstory]

Ran this locally before merging and it runs cleanly.