[REFACTOR] Phase out src/support/ffi_testing.cc (#19459)
Deletes src/support/ffi_testing.cc (271 lines) and removes TVM-only
testing symbols (TestAttrs, FrontendTestModule, TestingEventLogger,
ErrorTest). The duplicated testing.echo, testing.nop,
testing.object_use_count, and testing.run_check_signal symbols continue
to resolve through tvm-ffi which already registers them. Removes test
files that depend exclusively on deleted symbols.
**Test plan**: 185 passed / 77 skipped / 0 failed across covered suites
(tests/python/all-platform-minimal-test/, tests/python/ir/,
tests/python/runtime/test_runtime_error.py, tests/python/tirx-base/,
tests/python/contrib/). Build clean (cmake + ninja); pre-commit clean.
Tianqi Chen
410b4cf931