Makefile: change 'make install' to install standalone tests

make install wasn't very useful without also installing scripts for
the qemu command lines. I suspect 'make install' was never used.
Installing standalone tests could be useful though, so let's do
that instead.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>