tests/check-block: Skip iotests when sanitizers are enabled

The sanitizers (especially the address sanitizer from Clang) are
sometimes printing out warnings or false positives - this spoils
the output of the iotests, causing some of the tests to fail.
Thus let's skip the automatic iotests during "make check" when the
user configured QEMU with --enable-sanitizers.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190823084203.29734-1-thuth@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>