Changed the location of generate-base.sh

generate-base.sh is currently found in tests/integration/base/ yet a base/ directory already exists in tests/integration/project/elements/base/ (which only contains base-alpine.bst)

As it stands, when required, this script is run manually to create a alpine-base.tar.gz file which includes the sysroot of an alpine docker image used for testing thus it's location within the tests/ repo is completely arbitrary. So, for the sake of cleanliness I've moved it into the latter directory.