get the test runner and `zig test` working; enable behavior tests for stage2

[andrewrk]

test "example" {}

[nix-shell:~/dev/zig/build-release]$ ./zig-out/bin/zig test test.zig
thread 743 panic: reached unreachable code
/home/andy/dev/zig/src/type.zig:1988:21: 0x3d650a in type.Type.elemType (zig)
            else => unreachable,
                    ^
/home/andy/dev/zig/src/Sema.zig:6222:43: 0x695bcc in Sema.namedFieldPtr (zig)
        .Pointer => object_ptr.ty.elemType(),
                                          ^
/home/andy/dev/zig/src/Sema.zig:3213:30: 0x5cd028 in Sema.zirFieldPtr (zig)
    return sema.namedFieldPtr(block, src, object_ptr, field_name, field_name_src);
                             ^
/home/andy/dev/zig/src/Sema.zig:215:66: 0x4f3f13 in Sema.analyzeBody (zig)
            .field_ptr                    => try sema.zirFieldPtr(block, inst),
                                                                 ^
/home/andy/dev/zig/src/Sema.zig:105:29: 0x4f02ca in Sema.analyzeFnBody (zig)
    _ = try sema.analyzeBody(block, body);
                            ^
/home/andy/dev/zig/src/Module.zig:3615:27: 0x42b9c4 in Module.analyzeFnBody (zig)
    try sema.analyzeFnBody(&inner_block, func.zir_body_inst);
                          ^
/home/andy/dev/zig/src/Compilation.zig:1921:56: 0x420534 in Compilation.performAllTheWork (zig)
                        .queued => module.analyzeFnBody(decl, func) catch |err| switch (err) {
                                                       ^
/home/andy/dev/zig/src/Compilation.zig:1618:31: 0x41caef in Compilation.update (zig)
    try self.performAllTheWork();
                              ^
/home/andy/dev/zig/src/main.zig:2233:20: 0x3b32a2 in updateModule (zig)
    try comp.update();
                   ^
/home/andy/dev/zig/src/main.zig:1965:17: 0x352651 in buildOutputType (zig)
    updateModule(gpa, comp, hook) catch |err| switch (err) {
                ^
/home/andy/dev/zig/src/main.zig:192:31: 0x33d417 in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .zig_test);
                              ^
/home/andy/dev/zig/src/main.zig:141:20: 0x335860 in main (zig)
    return mainArgs(gpa, arena, args);
                   ^
/home/andy/dev/zig/lib/std/start.zig:458:37: 0x32e0fa in std.start.callMain (zig)
            const result = root.main() catch |err| {
                                    ^
/home/andy/dev/zig/lib/std/start.zig:400:12: 0x31225e in std.start.callMainWithArgs (zig)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/dev/zig/lib/std/start.zig:318:17: 0x311245 in std.start.posixCallMainAndExit (zig)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/dev/zig/lib/std/start.zig:244:5: 0x310fa2 in std.start._start (zig)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
Aborted (core dumped)

Instead it should run the tests. We already have simplified test runner logic when builtin.zig_is_stage2 is true, but there is also work that needs to be done in the stage2 frontend in order to make @import("builtin").test_functions work properly.

In order to close this issue, add test coverage for running the behavior tests with stage2 (but not all behavior tests have to be passing).

[andrewrk]

Done as of dc88864