API endpoint for testing webhook

[ethantkoenig]

Adds a POST /repos/:owner/:repo/hooks/:id/tests API endpoint for testing a push webhook. Also adds a unit test for the endpoint.

[codecov-io]

Codecov Report

Merging #3550 into master will decrease coverage by 3.06%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master    #3550      +/-   ##
==========================================
- Coverage   23.29%   20.23%   -3.07%     
==========================================
  Files         126      145      +19     
  Lines       25002    28997    +3995     
==========================================
+ Hits         5825     5867      +42     
- Misses      18289    22240    +3951     
- Partials      888      890       +2

Impacted Files	Coverage Δ
routers/api/v1/repo/hook.go	19.11% <76.47%> (ø)
routers/api/v1/repo/issue_comment.go	0% <0%> (ø)
routers/api/v1/repo/issue_label.go	0% <0%> (ø)
routers/api/v1/repo/fork.go	0% <0%> (ø)
routers/api/v1/repo/milestone.go	0% <0%> (ø)
routers/api/v1/repo/release.go	0% <0%> (ø)
routers/api/v1/repo/issue.go	0% <0%> (ø)
routers/api/v1/repo/release_attachment.go	0% <0%> (ø)
routers/api/v1/repo/file.go	0% <0%> (ø)
routers/api/v1/repo/branch.go	0% <0%> (ø)
... and 11 more

---

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[lunny]

LGTM

[lunny]

conflict

[ethantkoenig]

@lunny conflict fixed