In preparation to a following commit, add clang-format on and
clang-format off stanzas around constant definitions and the TEST_F_FORK
macro.  This enables to keep aligned values, which is much more readable
than packed definitions.

Add other clang-format exceptions for FIXTURE() and
FIXTURE_VARIANT_ADD() declarations to force space before open brace,
which is reported by checkpatch.pl .

Link: https://lore.kernel.org/r/20220506160513.523257-4-mic@digikod.net
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>