Better testify usage

[twz123]

What are you trying to accomplish?

Better UX in case of test failures:

• Better assertion messages
• No unneeded panics

Moreover: Make code related to test assertions a bit more concise.

What approach did you choose and why?

• Use assert.NoError instead of assert.Nil
  This gives nicer error messages.

• Swap arguments to assert.Equal
  The expectation comes first. Otherwise, error messages will be misleading.

• Use assert.Error instead of assert.NotNil
  This gives nicer error messages.

• Use assert.ErrorAs and assert.ErrorContains
  This simplifies the assertions and potentially gives better error messages.

• Use require instead of assert and use assert.NotNil as guard
  This is to prevent panics in tests, when things get accessed which shouldn't be nil.

Anything you want to highlight for special attention from reviewers?

An example of a misleading assertion message (expected and actual are flipped):

=== RUN   TestDeleteWithIncorrectRepoForDeleteCaches
Error: authentication token not found for host github.com
	delete_test.go:56:
				Error Trace:    /build/source/cmd/delete_test.go:56
				Error:          Should be true
				Test:           TestDeleteWithIncorrectRepoForDeleteCaches
				Messages:       1 unmatched mocks: https://api.github.com/repos/testOrg/testRepo/actions/caches?key=cacheName
	delete_test.go:59:
				Error Trace:    /build/source/cmd/delete_test.go:59
				Error:          Not equal:
								expected: "authentication token not found for host github.com"
								actual  : "The given repo does not exist."

								Diff:
								--- Expected
								+++ Actual
								@@ -1 +1 @@
								-authentication token not found for host github.com
								+The given repo does not exist.
				Test:           TestDeleteWithIncorrectRepoForDeleteCaches
--- FAIL: TestDeleteWithIncorrectRepoForDeleteCaches (0.00s)

[t-dedah]

Thank you @twz123 for the contribution. 💯

[t-dedah]

LGTM