admin: Fix tests locally, properly isolate storage

[francislavoie]

The admin tests were failing when I ran it locally:

--- FAIL: TestManageIdentity (0.00s)
    --- FAIL: TestManageIdentity/default_issuer_when_none_specified (0.00s)
        admin_test.go:919: Expected no error but got: localhost: caching certificate: tls: failed to find any PEM data in certificate input
    --- FAIL: TestManageIdentity/custom_issuer (0.00s)
        admin_test.go:919: Expected no error but got: localhost: caching certificate: tls: failed to find any PEM data in certificate input
FAIL

This is because we explicitly use DefaultStorage in admin.go for the local identity (which should not be in synced/clustered storage) but this caused storage overrides in tests to not work so it was using my own user's local storage in ~/.local/share/caddy when it shouldn't.

This adds a test-only local var to override the storage, and at runtime continues to use DefaultStorage as normal. Also improved test cleanup and isolation by using a temp storage instead of testdata/ since new files get written to it during the test.

Assistance Disclosure

Used GitHub Copilot to identify the problem in the tests and implement a fix.

[mholt]

Ah, thank you Francis, this looks great actually!