Fix standalone test simple/cat/main.zig after Writergate update

[imomaliev]

test/standalone/simple/cat/main.zig panics on run in version 0.15.1, but in 0.14.1 it works properly

/.../zig/std/debug.zig:559:14: 0x102e0fe07 in assert (...)
    if (!ok) unreachable; // assertion failure
             ^
/.../zig/0.15.1/lib/zig/std/Io/Writer.zig:939:11: 0x102ea2bb7 in sendFileAll (...)
    assert(w.buffer.len > 0);
          ^
/.../src/main.zig:35:39: 0x102ea28df in main (..)
            _ = try stdout.sendFileAll(&file_reader, .unlimited);
                                      ^

Steps to reproduce

$ zig init
# replace contents of src/main.zig with test/standalone/simple/cat/main.zig
$ zig build
$ echo "test" > t.txt
$ ./zig-out/bin/cat t.txt

I was not sure if the code in simple/cat should be actually working or only buildable, but this PR makes it work again

JFYI I did start discussion in ziggit, but decided to push fix to upstream at the end

[alexrp]

I wonder why we're apparently not building this test in CI? That seems unintentional.

[imomaliev]

@alexrp Hi! It works on build, it fails when trying to run the program. Maybe I was searching wrong, but I did not find anything in tests that actually checks if the program is runnable

[andrewrk]

Looks good, thank you!