integrate fuzz testing into the build system

[andrewrk]

Adds a --fuzz CLI option to the build runner. When this is used it rebuilds any unit test binaries which contained at least one fuzz test with -ffuzz and then tells it to start fuzzing, which does in-process fuzzing.

Adds std.testing.fuzzInput, which is how unit tests mark themselves as fuzz tests.

This contains only a rudimentary implementation of fuzzer logic, really just some early, early experiments, but already it makes this test case fail in 65 milliseconds on my machine:

test "fuzz example" {
    const input_bytes = std.testing.fuzzInput(.{});
    try std.testing.expect(!std.mem.eql(u8, "canyoufindme", input_bytes));
}

andy@bark ~/t/abc [1]> ~/dev/zig/build-release/stage4/bin/zig build test --fuzz --debug-rt
test
└─ run test failure
/home/andy/dev/zig/lib/std/testing.zig:546:14: 0x1183a59 in expect (test)
    if (!ok) return error.TestUnexpectedResult;
             ^
/home/andy/tmp/abc/src/main.zig:28:5: 0x1183b21 in test.fuzz example (test)
    try std.testing.expect(!std.mem.eql(u8, "canyoufindme", input_bytes));
    ^
failed with error.TestUnexpectedResult
error: the following command exited with error code 1:
/home/andy/tmp/abc/.zig-cache/o/15697f80a9140388803537517083788c/test --seed=0x93a767fd --listen=- 
error: all fuzz workers crashed
error: the following build command failed with exit code 1:
/home/andy/tmp/abc/.zig-cache/o/12ab17aae32a6d5d6c9fba7b28ba3167/build /home/andy/dev/zig/build-release/stage4/bin/zig /home/andy/dev/zig/lib /home/andy/tmp/abc /home/andy/tmp/abc/.zig-cache /home/andy/.cache/zig --seed 0x93a767fd -Zb1f080d4c8ac26d5 test --fuzz --debug-rt

asciinema demo

Closes #20702.

Follow-Up Tasks

• false positive cache hit on compiler_rt and libfuzzer when used via the build system #20782
• support code coverage when testing #352
• Proposal: Add a way for a test to expect a panic #1356
• Consider running tests in parallel by deafult #15953
• unit tests: enforce time limits #19821
• write fuzz inputs to a shared memory region before running a task #20803
• enhance the fuzzing algorithm to be competitive with other mainstream fuzzers #20804
• add a UI to fuzzing to report progress, code coverage, interesting inputs, and other stats #20812
• std.testing.fuzzInput: introduce a corpus option #20814
• std.testing.fuzzInput: ability to choose whether timing out is considered a fail or a pass #20815
• std.testing.fuzzInput: introduce a length range option #20816
• unique failure detection
• corpus minimization mode
• more flags to the build runner such as timed fuzzing
• investigate adding sometimes assertions, perhaps with scopes
• investigate coordination between multiple zig build --fuzz instances running on different machines
• integrate --fuzz with --watch

[rohlem]

• investigate adding sometimes assertions, perhaps with scopes

In case these get added I want to propose the name assertReachable.