LU-18953 tests: verify identity_upcall is executable

LU-18953 tests: verify identity_upcall is executable

In test-framework, verify that the value of the L_GETIDENTITY
variable points to an executable. Otherwise, the identity_upcall
would fail.

Test-Parameters: trivial testlist=conf-sanity
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I5839c157c3678caa81908f950d3cc2cd2fc59c44
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58961
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>