On Linux systems that have statx available, stat_dev() returned
stx_dev_major, which could be the same value for different partitions.
Change it to combine stx_dev_major and stx_dev_minor to get a unique
number (meaning a unique UDSEntry::UDS_DEVICE_ID).
Extend JobTest::stat() unit test to check that UDS_DEVICE_ID is unique
for each individual partition.
Also in JobTest::stat(), use a smaller #if/#endif to keep the HTTP stat
bits disabled (it's been disabled since kdelibs4 split so...).