LU-15887 test: add always_except()

LU-15887 test: add always_except()

In test-framework.sh, add a new function (always_except()) to replace
manual manipulation of $ALWAYS_EXECPT. Convert sanity.sh to use
always_except() and add a line to contrib/scripts/spelling.txt to
suggest its use.

Test-Parameters: trivial
Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I1b39fe9555bab59e70db00cef73d13102668500a
Reviewed-on: https://review.whamcloud.com/47452
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>