kselftest/arm64: signal: fix/refactor SVE vector length enumeration

kselftest/arm64: signal: fix/refactor SVE vector length enumeration

Currently a number of SVE/SME related tests have almost identical
functions to enumerate all supported vector lengths. However over time
the copy&pasted code has diverged, allowing some bugs to creep in:
- fake_sigreturn_sme_change_vl reports a failure, not a SKIP if only
  one vector length is supported (but the SVE version is fine)
- fake_sigreturn_sme_change_vl tries to set the SVE vector length, not
  the SME one (but the other SME tests are fine)
- za_no_regs keeps iterating forever if only one vector length is
  supported (but za_regs is correct)

Since those bugs seem to be mostly copy&paste ones, let's consolidate
the enumeration loop into one shared function, and just call that from
each test. That should fix the above bugs, and prevent similar issues
from happening again.

Fixes: 4963aeb35a9e ("kselftest/arm64: signal: Add SME signal handling tests")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240821164401.3598545-1-andre.przywara@arm.com
Signed-off-by: Will Deacon <will@kernel.org>