This switches the exec test to `SIGTERM` rather than `SIGQUIT` since the
latter didn’t seem to work in QEMU, and the distinction doesn’t really
matter for this test.
This also makes the `isatty()` check smarter by checking whether `STDIN`
is, in fact, a terminal.
Added qemu-alpine targets i386, arm32v6, arm32v7, arm64v8, s390x
Co-authored-by: Felipe Gasper <felipe@felipegasper.com>