test: correct architecture in EFI FIT test

On arm64 the its we use to generate the test FIT image has

	arch = "arm";

We should use "arm64" here which is mapped to IH_ARCH_ARM64 via
uimage_arch[].

Fixes: 8391f955 ("test/py: Create a test for launching UEFI binaries from FIT images")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>