debian/tests/control: Mark all amd64 tests as skip-not-installable

As these tests require installing an amd64 kernel package, they have
to be skipped on other architectures.  Tthere is currently no way to
explicitly restrict them to specific architectures, so instead add the
skip-not-installable restriction.

Closes: #970184
Signed-off-by: Ben Hutchings <benh@debian.org>