[vm/ffi] Migrate legacy multi test files

Running the test suite emits a bunch of warnings.

2025-02-26 09:26:32.618061 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/vmspecific_enable_ffi_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.632282 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/vmspecific_function_callbacks_exit_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.638006 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/unaligned_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.640032 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/vmspecific_highmem_32bit_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.663762 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/regress_47673_2_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.664344 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/vmspecific_variance_function_checks_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.665567 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/regress_44986_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.679988 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/regress_46085_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.681193 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/vmspecific_static_checks_ffinative_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.683955 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/vmspecific_static_checks_varargs_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.684641 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/regress_51041_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.685302 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/vmspecific_regress_38993_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.685744 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/static_checks/vmspecific_function_callbacks_negative_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.691609 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/vmspecific_leaf_call_test.dart is a legacy multi-test file.
2025-02-26 09:26:32.727318 Warning: /Users/dacoharkes/dart-sdk/sdk/tests/ffi/abi_specific_int_incomplete_aot_test.dart is a legacy multi-test file.

• The static_checks files should be migrated to the // [cfe] ... && // [analyzer] ... expectations.
• For the other files:
  • The //# 01: ok can be simply removed to make it a non-multi-test. https://dart-review.googlesource.com/c/sdk/+/412240
  • The compile-time errors either need to be moved to // [cfe] ... if the compile-time error is emitted in the CFE or to a test spawning a new isolate from uri and checking for the compile-time error in there.

https://github.com/dart-lang/sdk/blob/main/runtime/docs/contributing_to_dart_ffi.md