test: a few tweaks regarding setup of test-environment

• have image-files in /dev/shm to assure they are in memory and sbd opening the files with O_SYNC doesn't trigger unnecessary syncs on a heavily loaded test-machine fallback to /tmp if /dev/shm doesn't exist
• wrapping away libaio and usage of device-mapper for block-device simulation can now be passed into make via SBD_USE_DM & SBD_TRANSLATE_AIO
• have variables that configure test-environment be printed out prior to running tests
• finally assure we clean environment when interrupted by a signal (bash should have done it with just setting EXIT handler - but avoiding bashism might come handy one day)