blktests: Use old variable check for Bash <4.2

blktests: Use old variable check for Bash <4.2

Bash 4.2 and above supports -v variable checks, which returns true for
set or null. Instead use an older bashism that is compatible with bash
4.1 and earlier but only returns true if the variable is set non-null.
This inherently adds a sanity check in case of null variables.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>