debian/tests: restrict openblas tests to available architectures

restrict openblas tests to only those architectures
for which openblas is available.

Architecture: amd64 arm64 armhf i386 powerpc ppc64el ppc64
kfreebsd-i386 kfreebsd-amd64 mips64el riscv64 sparc64 s390x

i.e. not armel