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

In ./src/test/fuzz/fuzz_multi.sh line 1:
MEMLIMIT_BYTES=21990500990976
^-- SC2148: Tips depend on target shell and yours is unknown. Add a shebang.


In ./src/test/fuzz/fuzz_multi.sh line 9:
FILTER=echo
^----^ SC2209: Use var=$(command) to assign output (or quote to assign string).


In ./src/test/fuzz/fuzz_multi.sh line 11:
for i in `seq -w "$N_CPUS"`; do
         ^----------------^ SC2006: Use $(...) notation instead of legacy backticked `...`.

For more information:
  https://www.shellcheck.net/wiki/SC2148 -- Tips depend on target shell and y...
  https://www.shellcheck.net/wiki/SC2209 -- Use var=$(command) to assign outp...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...