Integration tests

[Ilia-Kosenkov]

As was discussed in #15, without proper tests it is impossible to catch errors like the one fixed in #16.
However, tests cannot be rust-dependent or otherwise compromise CRAN builds.
So tests can be either skipped on CRAN (or using other condition), or they can be implemented outside of the normal test framework and executed manually on CI.

I suggest to have skip_on_cran tests that can be executed when package is installed by the client -- to immediately reveal if client's system is misconfigured.

[clauswilke]

We currently skip all tests when cargo is not available:

rextendr/tests/testthat.R

Lines 5 to 8 in fa8dd4b

	result <- system2("cargo")
	if (result == 0L) {
	test_check("rextendr")
	}

This check should be extended to skip all tests on CRAN. I don't think there are going to be any tests that meaningfully can be executed on CRAN.

The check should use something like this code from testthat:
https://github.com/r-lib/testthat/blob/149c888b8163c7070d66d691c2ef6d7b3eed8745/R/skip.R#L131

[clauswilke]

It's probably a good idea to merge #7 before addressing this issue.

[yutannihilation]

Can we close this issue?

[clauswilke]

Yes, I think so. I'll mention that maybe we need to rethink the idea of skipping all tests on CRAN. We're starting to add many unit tests that aren't Rust-dependent and that could run on CRAN.