checkAllAllocationFailures: provide result type to 'extra_args'

[Techatrix]

The removal of anonymous struct types (#21817) has broken the following usage of std.testing.checkAllAllocationFailures:

const std = @import("std");
test {
    try std.testing.checkAllAllocationFailures(
        std.testing.allocator,
        struct {
            fn f(ally: std.mem.Allocator, params: struct {
                foo_len: u32,
                bar_len: u32,
            }) !void {
                _ = ally;
                _ = params;
            }
        }.f,
        .{
            .{
                .foo_len = 3,
                .bar_len = 5,
            },
        },
    );
}

The signature of checkAllAllocationFailures looks like this:

pub fn checkAllAllocationFailures(
    backing_allocator: std.mem.Allocator,
    comptime test_fn: anytype,
    extra_args: anytype,
) !void

The parameter types of test_fn are not provided as result types to extra_args which means that it will construct a "normal" struct type that won't be able to coerce to the @TypeOf(params) which will cause a compile error similar to this:

lib/std/testing.zig:1097:35: error: expected type 'sample.test_0__struct_1680.f__struct_1681', found 'sample.test_0__struct_1688'
        @field(args, arg_i_str) = @field(extra_args, field.name);
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sample.zig:15:14: note: struct declared here
            .{
            ~^
sample.zig:6:51: note: struct declared here
            fn f(ally: std.mem.Allocator, params: struct {
                                                  ^~~~~~

This fix should also improve the compile error messages if the provide extra arguments are incorrect because the validation is performed at the callsite instead of inside of the checkAllAllocationFailures function.

I have also tested this fix with https://github.com/ziglibs/diffz which frequently uses this code pattern in its tests.