test: add Mongo Asset's unit testing

[KeisukeYamashita]

Overview

I have added initial unit testing for MongoDB based on this issue 👉 reearth/reearth-visualizer#273.
See details (requirements) in that issue.

I implemented one for a trial to avoid conflicts and decide how we are going to test this method.

What I've done

I have added a unit test for the Asset repository's Remove method.

What I haven't done

Add unit tests for other methods.

How I tested

The testings are done by the CI GitHub Actions workflow.
This can be also tested locally.

# Skip testing
go test -timeout 30s -run '^TestFindByID$' github.com/reearth/reearth-backend/internal/infrastructure/mongo

# Test (after launching MongoDB instance in your local environment)
REEARTH_DB=mongodb://localhost go test -timeout 30s -run '^TestFindByID$' github.com/reearth/reearth-backend/internal/infrastructure/mongo

Which point I want you to review particularly

• How the testing(s) should be done

Memo

FYI: This PR is on top of this PR #139

[codecov]

Codecov Report

Merging #141 (9b3ac0e) into main (4f72b87) will decrease coverage by 2.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
- Coverage   36.68%   34.64%   -2.04%     
==========================================
  Files         331      349      +18     
  Lines       29890    31755    +1865     
==========================================
+ Hits        10964    11001      +37     
- Misses      17925    19752    +1827     
- Partials     1001     1002       +1     

Impacted Files	Coverage Δ
internal/infrastructure/mongo/property_schema.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/project.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/scene.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/auth_request.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/team.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/config.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/transaction.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/tag.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/dataset.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/plugin.go	0.00% <0.00%> (ø)
... and 8 more