document workaround test_bp2build_generates_all_buildfiles

test_bp2build_generates_all_buildfiles uses the same directory for
--output_base and --sandbox_tmpfs_path. This is required on the build
servers, but causes issues locally. This commit adds a more helpful
error message when this test fails.

Test: build/soong/tests/bp2build_bazel_test.sh
Change-Id: Ib29a7edf456f8ae39bd429ebb367c8115e02c284