multipath: work around false shellcheck error

multipath: work around false shellcheck error

Shellcheck seems to think that [ -z "$debug" ] is setting the debug
variable and warns:

  common/multipath-over-rdma:190:11: note: Modification of debug is local (to subshell caused by pipeline). [SC2030]
  common/multipath-over-rdma:606:8: note: debug was modified in a subshell. That change might be lost. [SC2031]

Work around this by using test instead.

Signed-off-by: Omar Sandoval <osandov@fb.com>