Fix running t/getopt.t from a read-only location

[ppisar]

If current working directory is a read-only location, running t/getopt.t fails:

$ perl t/getopt.t
1..14
ok 1
ok 2
ok 3
ok 4
ok 5
Writing to config file <getopt.tst> failed: Permission denied
 at t/getopt.t line 62.

This patch fixes it by using a temporary directory with File::Temp. File::Temp is smart enough to find a suitalbe writable location and clean up on exit. File::Temp is already used in other tests, e.g. in t/modern.t.