block/011: set default timeout to 20 minutes

block/011: set default timeout to 20 minutes

The test case runs fio while disabling and enabling PCI device of the
test target block device. Depending on the device type, it takes very
long time to re-enable the device. At worst case, it takes 4 hours to
complete the test case.

To avoid the meaningless long test runtime, set default timeout limit. I
ran the test case on various devices: real NVME SSD, QEMU NVME
emulation, HDDs with AHCI, HDDs with SAS-HBA. Many of them takes less
than 20 minutes to complete and pass the test case. Hence, choose 20
minutes as the timeout duration.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>