test_dtb: accept additional 'v' in fdtump --version

CI fails with:
tests/comparators/test_dtb.py:45: in fdtdump_version
    raise ValueError("Error parsing `fdtdump --version` output")
E   ValueError: Error parsing `fdtdump --version` output
        m          = None
        out        = ('\n'
 '**** fdtdump is a low-level debugging tool, not meant for general use.\n'
 '**** If you want to decompile a dtb, you probably want\n'
 '****     dtc -I dtb -O dts <filename>\n'
 '\n'
 'Version: DTC v1.7.2-1-g9af601c\n')

Maybe Arch overrides the version or something. Let's just skip the 'v'
if it is there.