This unifies the test scripts to all use the similar pattern added for
schematests in ace4aecd. This gives the following

- Enables running all tests from outside of tests/ dir
- Drops redundant abs_* definitions, which are set by test-lib.sh
- Drops unnecessary srcdir variable which was only used for sourcing
    test-lib.sh

Behavior changes:

- srcdir can no longer be overwritten, but I don't know why anyone would
    really need to...
- Script VERBOSE setting no longer prints commands executed by test-lib.sh.
    if anyone cares I suggest handling this in test-lib.sh which already
    has other verbose style handling

Commit a0b6a36f is fixing what commit abfff210 broke, so to avoid having
to deal with this issue again, herec comes "virsh-uriprecedence".