test: find $BUILD_DIR in test-functions, remove from other scripts

test: find $BUILD_DIR in test-functions, remove from other scripts

The $BUILD_DIR is only used in test-functions, and doesn't need to
be specified in any other scripts. Additionally, to be able to allow
the integration test suite to be run against locally installed binaries,
instead of built binaries, moving BUILD_DIR logic completely into
test-functions allows later patches to be simpler.