Regression test for #273

The cache status of reverse dependencies of workspaced elements cannot
always be determined upfront.

These tests are too long to understand at a glance so a summary is
needed of each.

Using shlex messes with the argv we want to pass to a shell, this
was not working for the newly added test which passes "${var}" strings
as arguments to the shell.

Also, removed a redundant line to explicitly build something
in the 'no_shell' test; that is taken care of by the helper.

When testing a recent patch, I had missed a return of the exit code
in `bst shell` resulting in the CLI exiting with `None`; this patch
makes the fixture more helpful, where previously it just printed `None`

We have been using the Freedesktop SDK binaries to provide a base
system to run the integration tests. This works OK but it weighs in
at 985MB of content, and there is no simple way to customize it so
that we only download the bits we actually need.

This commit changes the tests to use a custom sysroot based on the
Alpine Linux distribution. The sysroot is 155MB unpacked, and packs down
to a 27MB .tar.xz.

This speeds up the integration tests significantly as we greatly reduce
the amount of network traffic required and the amount of data that gets
copied around when creating the staging area.

This means that the integration test suite no longer requires a C++
compiler, and in practice this can allow us to avoid depending on the
GNU C/C++ compilers altogether and instead use the vastly smaller Tiny C
Compiler.