Retry test-fixtures loading in case of transaction rollback

[typeless]

In some circumstances, the transaction in the test fixture loading function would rollback when a transaction conflict occurs.
Retry it five times to at least reduce the chance of failures.

[codecov-io]

Codecov Report

Merging #5125 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #5125      +/-   ##
=========================================
- Coverage    37.4%   37.4%   -0.01%     
=========================================
  Files         308     308              
  Lines       45605   45612       +7     
=========================================
+ Hits        17057   17059       +2     
- Misses      26091   26094       +3     
- Partials     2457    2459       +2

Impacted Files	Coverage Δ
models/test_fixtures.go	100% <100%> (ø)	⬆️
models/repo_indexer.go	50.84% <0%> (-2.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2227446...e323a13. Read the comment docs.