refactor: remove gotest.tools

[catthehacker]

• remove all references to gotest.tools and replace it with
  github.com/stretchr/testify which was originally used for tests
• bump golangci-lint version
• add depguard and importas to prevent import of unwanted packages
• add custom schema and information about config since
  schemastore.org has broken schema for golangci-lint config

[codecov]

Codecov Report

Merging #688 (87c1dc1) into master (0f04942) will increase coverage by 1.24%.
The diff coverage is 60.70%.

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
+ Coverage   49.27%   50.51%   +1.24%     
==========================================
  Files          23       23              
  Lines        2401     2532     +131     
==========================================
+ Hits         1183     1279      +96     
- Misses       1090     1114      +24     
- Partials      128      139      +11     

Impacted Files	Coverage Δ
pkg/container/docker_run.go	1.82% <0.00%> (-0.11%)	⬇️
pkg/model/workflow.go	30.71% <33.33%> (+5.00%)	⬆️
pkg/common/git.go	55.12% <33.92%> (-4.68%)	⬇️
pkg/model/planner.go	34.56% <41.37%> (+1.48%)	⬆️
pkg/container/docker_pull.go	36.17% <64.70%> (+17.98%)	⬆️
pkg/runner/step_context.go	71.99% <71.31%> (+3.04%)	⬆️
pkg/runner/run_context.go	79.74% <98.18%> (+3.33%)	⬆️
pkg/runner/runner.go	76.92% <100.00%> (+0.45%)	⬆️
... and 2 more

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[mergify]

@catthehacker this pull request has failed checks 🛠