Add a test suite ✅

This adds a test suite to be executed using `linuxkit/rtf`.
This is installed in the top-level Makefile

The tests are written in shell script and cover the following cases:

- Kernel Config is OK!
- Kernel Modules can be built and loaded
- QEMU can build and run kernel+initrd, iso-bios and iso-uefi
- That we can build for all other supported output formats
- That all of the examples in `./examples` can be built
- The LTP tests can be run (if `-l slow` is provided)

The virtsock and docker-bench tests were migrated but no test has been
written as yet as AFAICT they are still a WIP

Signed-off-by: Dave Tucker <dt@docker.com>

test/cases/_lib/lib.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# Source the main regression test library if present
+[ -f "${RT_LIB}" ] && . "${RT_LIB}"
+
+# Temporary directory for tests to use.
+LINUXKIT_TMPDIR="${RT_PROJECT_ROOT}/_tmp"
+LINUXKIT_ARTIFACTS_DIR="${RT_PROJECT_ROOT}/../../artifacts"
+
+# The top-level group.sh of the project creates a env.sh file
+# containing environment variables for tests. Source it if present.
+[ -f "${LINUXKIT_TMPDIR}/env.sh" ] && . "${LINUXKIT_TMPDIR}/env.sh"
+
+# FIXME: Should source the rtf/lib/lib.sh instead
+RT_CANCEL=253