remove duplicate profile threshold tests from add_test.go since they
are fully covered by the data-driven tests in cid_profiles_test.go.
changes:
- improve test names to describe what threshold is being tested
- add inline documentation explaining each test's purpose
- add byte-precise helper IPFSAddDeterministicBytes for threshold tests
- remove ~200 lines of duplicated test code from add_test.go
- keep non-profile tests (pinning, symlinks, hidden files) in add_test.go
