shellcheck: src/test/fuzz/fixup_filenames.sh issues

In ./src/test/fuzz/fixup_filenames.sh line 11:
    prev=`basename "$fn"`
         ^--------------^ SC2006: Use $(...) notation instead of legacy backticked `...`.


In ./src/test/fuzz/fixup_filenames.sh line 12:
    post=`sha256sum "$fn" | sed -e 's/ .*//;'`
         ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.


In ./src/test/fuzz/fixup_filenames.sh line 13:
    if [ "$prev" == "$post" ] ; then
                 ^-- SC2039: In POSIX sh, == in place of = is undefined.

For more information:
  https://www.shellcheck.net/wiki/SC2039 -- In POSIX sh, == in place of = is ...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...