Add simple tests of builder UID.

tests/integration/build-uid: Add tests.
tests/integration/project/elements/build-uid/build-uid-default.bst:
    A test for the default uid/gid of 0 in the sandbox.
tests/integration/project/elements/build-uid/build-uid.bst:
    Tests for nonzero uid/gid values supplied in the element.
tests/integration/project/elements/build-uid/build-uid-1023.bst:
    Tests for nonzero uid/gid values supplied in project.conf.