chore: Fix tracking-issue tests

Created by: efritz

Looks like file modtime is not going to be stable across machines. I'm adding an explicit file with the last update time instead.

Created by: codecov[bot]

Codecov Report

Merging #14172 into main will increase coverage by 0.62%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #14172      +/-   ##
==========================================
+ Coverage   50.84%   51.47%   +0.62%     
==========================================
  Files         763     1514     +751     
  Lines       22731    77112   +54381     
  Branches     6942     6879      -63     
==========================================
+ Hits        11557    39690   +28133     
- Misses      11117    33839   +22722     
- Partials       57     3583    +3526     

Flag	Coverage Δ
#go	51.72% <100.00%> (?)
#integration	29.20% <ø> (+0.01%)
#storybook	18.32% <ø> (ø)
#typescript	50.85% <ø> (+<0.01%)
#unit	34.04% <ø> (ø)

Impacted Files	Coverage Δ
internal/cmd/tracking-issue/util.go	58.46% <100.00%> (ø)
enterprise/cmd/frontend/auth/saml/session.go	0.00% <0.00%> (ø)
internal/extsvc/phabricator/client.go	88.03% <0.00%> (ø)
cmd/frontend/internal/session/session.go	35.21% <0.00%> (ø)
enterprise/internal/codeintel/store/dumps.go	87.61% <0.00%> (ø)
cmd/frontend/graphqlbackend/repository_git_refs.go	0.00% <0.00%> (ø)
internal/leader/leader.go	86.66% <0.00%> (ø)
enterprise/internal/codeintel/resolvers/util.go	0.00% <0.00%> (ø)
...terprise/cmd/frontend/internal/registry/mock_db.go	100.00% <0.00%> (ø)
cmd/searcher/search/filter.go	41.66% <0.00%> (ø)
... and 743 more