[tests] Add tests for the \plugin:: syntax for import

tests/spec/Extend_conflict.i

tests/spec/Extend_errors.c

tests/spec/Extend_errors.ml

tests/spec/Extend_kernel.i

tests/spec/Extend_plugin.ml

tests/spec/Extend_recursive_preprocess.ml

tests/spec/Extend_type_token.i

tests/spec/Extend_warning.i

tests/spec/import.i

tests/spec/import_errors.c

tests/spec/module.c

tests/spec/oracle/Extend_conflict.res.oracle

tests/spec/oracle/Extend_errors.0.res.oracle

tests/spec/oracle/Extend_errors.1.res.oracle

tests/spec/oracle/Extend_errors.2.res.oracle

tests/spec/oracle/Extend_errors.3.res.oracle

tests/spec/oracle/Extend_errors.4.res.oracle

tests/spec/oracle/Extend_kernel.res.oracle

tests/spec/oracle/Extend_type_token.res.oracle

tests/spec/oracle/Extend_warning.res.oracle