pkg/fuzzer: collect executor debug logs in tests

[a-nogikh]

It should hopefully let us debug #5674.

(Currently we actually don't return the error in that case, but it will change once #5700 is merged).

[a-nogikh]

Haha, that happened fast:

--- FAIL: TestFuzz (11.22s)
    testutil.go:35: seed=0
Error: seed=0
    fuzzer_test.go:184: executor output:
Error: executor output:
        connected to manager: procs=4 cover_edges=1 kernel_64_bit=1 slowdown=1 syscall_timeout=50 program_timeout=5000 features=0x10
        received info reply: covfilter=0
        recv exec request 1: type=0 flags=0x3 env=0x1c09 exec=0x10 size=14
        proc 0: handshaking to execute request 1
        proc 0: got handshake reply
        proc 0: start executing request 1
        proc 0: got output: [9ms] exec opts: reqid=1 type=0 procid=0 threaded=1 cover=0 comps=0 dedup=0 signal=0  sandbox=1/0/0/0 timeouts=50/5000/1 kernel_64_bit=1
        proc 0: got output: spawned worker pid 90923
        proc 0: got output: Fatal glibc error: malloc assertion failure in sysmalloc: (old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)
        proc 0: got output: Fatal glibc error: malloc assertion failure in sysmalloc: (old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)
        proc 0: got execute reply
        handle completion: completed=0 output_size=262144
        proc 0: output pipe EOF
        proc 0: restarting subprocess, current state 2 attempts 0
        proc 0: subprocess exit status 6
        SYZFAIL: failed to recv rpc
        fd=3 want=4 recv=0 n=0 (errno 9: Bad file descriptor)
    fuzzer_test.go:185: executor process exited: exit status 67