### Describe Your Changes
This PR continues the implementation of integration tests (#7199). It
adds the support for vm-single:
- A vmsingle app wrapper has been added
- Sample vmsingle tests that test the VM documentation related to
querying data (#7435)
- The tests use the go-cmp/{cmp,/cmpopts} packages, therefore they have
been added to ./vendor
- Minor refactoring: data objects have been moved to model.go
Advice on porting things to cluster branch:
- The build rule must include tests that start with TestVmsingle
(similarly to how TestCluster tests are skipped in master branch)
- The build rule must depend on `vmstorage vminsert vmselect` instead of
`victoria-metrics`
- The query_test.go can actually be implemented for cluster as well. To
do this the tests need to be renamed to start with TestCluster and the
tests must instantiace vm{storage,insert,select} instead of vmsingle.
### Checklist
The following checks are **mandatory**:
- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
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Signed-off-by: Artem Fetishev <rtm@victoriametrics.com>
Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>
