chore: replace log.Fatal with t.Fatal in tests

[alexandear]

This PR improves tests by replacing log.Fatal with t.Fatal. log.Fatal immediately interrupts all tests and call os.Exit(1), which does not run any deferred statements. t.Fatal interrupt only failed test, shows the line with failure, and ensures all deferred statements are run.

Compare outputs:

1. log.Fatal:

=== RUN   TestGetIssue
2024/11/25 14:28:15 some error

2. t.Fatal:

=== RUN   TestGetIssue
    /Users/Oleksandr_Redko/src/github.com/xanzy/go-gitlab/issues_test.go:37: some error
--- FAIL: TestMoveIssue (0.00s)

This was found with revive.deep-exit rule.

[RicePatrick]

Looks good; great change! Thanks @alexandear 🎉