tests: make 'cross test' work
tests: make 'cross test' work

The reason why it wasn't working was the integration tests. Namely, the
integration tests attempted to execute the 'rg' binary directly from
inside cross's docker container. But this obviously doesn't work when
'rg' was compiled for a totally different architecture.

Cross normally does this by hooking into the Rust test infrastructure
and causing tests to run with 'qemu'. But our integration tests didn't
do that. This commit fixes our test setup to check for cross's
environment variable that points to the 'qemu' binary. Once we have
that, we just use 'qemu-foo rg' instead of 'rg'. Piece of cake.

BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep