oeqa/selftest/wic: skip more tests on aarch64

test_rawcopy_plugin_qemu and test_wic_image_type are x86-specific
currently as the .wks uses x86-specific bootloaders.

This can be fixed, but that can come later.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>