Enable offline cassette replay for VCR-backed tests (#161)

* Remove remaining context() calls

* Define node test fixtures at file level

* Simplify osf_tbl stacking test

* Define upload test fixture at file-level

* Replace skip_on_production_server() with appropriate skip helpers

Rename skip_on_production_server() to skip_if_not_test_server() for
tests that depend on pre-existing assets on test.osf.io (test-osf_ls),
and switch to skip_if_no_pat() for tests that require authentication
(test-osf_tbl).

* Add skip_if_not_test_server() and README

Add skip_if_not_test_server() for tests that depend on pre-existing
assets on test.osf.io. Apply it to test-downloading.R (alongside
skip_if_no_pat) and the test-osf_tbl "can't combine" test that uses a
hardcoded test server GUID.

Add a README documenting the three testing modes (offline with
cassettes, live test server, live production server) and the skip
helpers.

* Add VCR cassettes for osf_ls and osf_tbl

Wrap API calls in vcr::use_cassette() so these tests replay recorded
responses instead of requiring a live connection to test.osf.io. This
brings them in line with the rest of the test suite.

* Enable offline cassette replay for VCR-backed tests

Remove `if (has_pat())` guards from setup blocks and `skip_if_no_pat()`
from individual tests so VCR cassettes replay without credentials.
Teardown blocks that conditionally clean up live resources are kept
unchanged. Also remove the unnecessary skip from the pure `id_type()`
test in test-identifiers.R.

Offline results improve from ~30 pass / ~74 skip to 150 pass / 9 skip,
with all remaining skips in test-downloading.R (which requires live
file access).

Author: aaronwolen