go.bbclass: don't stage test data with sources of dependencies

As for the sources the dependencies contain test data, ELF files and other
binaries which aren't necessary for building and which lead to unnecessary QA
warnings.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>