Add more benchmark tests to vec.rs

New benchmark tests in vec.rs:
`push`, `starts_with_same_vector`, `starts_with_single_element`,
`starts_with_diff_one_element_end`, `ends_with_same_vector`,
`ends_with_single_element`, `ends_with_diff_one_element_beginning` and
`contains_last_element`
auto merge of #10582 : g3xzh/rust/master, r=cmr

More new benchmark tests. some of them are benchmarking `starts_with` and `ends_with`.
Let me know if I am missing something.
Thanks in advance.