fusefs: Coverity cleanup in the lseek tests

Always check the return value of open().

Reported by: Coverity Scan
CID: 1471118 1471133 1471215 1471896 1471901 1472116 1473799
CID: 1473879 1473996 1555269 1558044
Sponsored by: ConnectWise

(cherry picked from commit f415b2ef30f7bf0db753f09fbba7b0910475b0d2)