feat: Add a helper to run both R and Rust tests with `devtools::test()`

[JosephBARBIERDARNAL]

It would be useful to have an exported helper / hook (I'm unsure how those are called), e.g. devtools::test()/rextendr::test(), that runs both the Rust and R test suites for an extendr package.

Currently this requires running two commands:

processx::run("cargo", "test", wd = "src/rust", echo = TRUE)
devtools::test()

A single command would make the development workflow more convenient and provide a standard way for extendr packages to run their complete test suite. This could either be devtools::test() or rextendr::test(), but the latter is less good IMO.

Related discussion: #509.