check-block: enable iotests with SafeStack

check-block: enable iotests with SafeStack

SafeStack is a stack protection technique implemented in llvm. It is
enabled with a -fsanitize flag.
iotests are currently disabled when any -fsanitize option is used,
because such options tend to produce additional warnings and false
positives.

While common -fsanitize options are used to verify the code and not
added in production, SafeStack's main use is in production environments
to protect against stack smashing.

Since SafeStack does not print any warning or false positive, enable
iotests when SafeStack is the only -fsanitize option used.
This is likely going to be a production binary and we want to make sure
it works correctly.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-id: 20200529205122.714-5-dbuono@linux.vnet.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>