doc: add cargo test --doc --features __macro-docs near-sdk/src/near.rs to test job

[dj8yfo]

for snippets in this module https://github.com/near/near-sdk-rs/blob/master/near-sdk/src/lib.rs#L54-L55 add
--features __macro-docs to test job , so they're tested and kept in sync.
currently they're correct:

❯ : cargo test --doc --features __macro-docs near-sdk/src/near.rs

running 5 tests
test near-sdk/src/near.rs - near::private (line 70) ... ok
test near-sdk/src/near.rs - near::init (line 13) ... ok
test near-sdk/src/near.rs - near::handle_result (line 125) ... ok
test near-sdk/src/near.rs - near::payable (line 42) ... ok
test near-sdk/src/near.rs - near::result_serializer (line 97) ... ok
                                                                                
test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 269 filtered out; finished in 1.00s

this is if these snippets are decided to be kept in near-sdk/src/lib.rs in scope of #1262 ,
and not moved to #[near] documentation