iotests: exclude killed processes from running under Valgrind

iotests: exclude killed processes from running under Valgrind

 The Valgrind tool fails to manage its termination in multi-threaded
 processes when they raise the signal SIGKILL. The bug has been reported
 to the Valgrind maintainers and was registered as the bug #409141:
 https://bugs.kde.org/show_bug.cgi?id=409141
 Let's exclude such test cases from running under the Valgrind until a
 new version with the bug fix is released because checking for the
 memory issues is covered by other test cases.

Suggested-by: John Snow <[email protected]>
Signed-off-by: Andrey Shinkevich <[email protected]>
Reviewed-by: John Snow <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>