kselftest/arm64: signal: Add a TODO list for signal handling tests

Note down a few gaps in our coverage.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210819134245.13935-7-broonie@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/signal/testcases/TODO

+- Validate that register contents are saved and restored as expected.
+- Support and validate extra_context.