Add execution time support

[EFanZh]

Recently, Rust has merged a --report-time flag (rust-lang/rust#64663) into the test library so that the execution time is printed. I think maybe cargo2unit can support this feature.

[johnterickson]

@EFanZh Can you pull master and try it out? I'd love a second pair of eyes before I publish 😊

[EFanZh]

I have tried the latest cargo2junit with nightly Rust, the result looks good to me. But I’ll have to wait about three months before I can use this feature with stable Rust.

[johnterickson]

Published - for those on nightly add you say 😁