lib/std/posix: test ftruncate via std.fs.File.setEndPos()

[rootbeer]

Add a test for std.fs.File's setEndPos (which is a simple wrapper around std.posix.ftruncate) to exercise some success and failure paths.

Explicitly check that the ftruncate length isn't negative when interpreted as a signed value. This avoids having to decode overloaded EINVAL errors.

Add errno handling to Windows path to map INVALID_PARAMETER to FileTooBig.

Fixes #22960

[squeek502]

Not super important, but might be nice to use testWithAllSupportedPathTypes

[rootbeer]

Not super important, but might be nice to use testWithAllSupportedPathTypes

I thought about that, but I think its not necessary because the path of the file isn't relevant. setEndPos is implemented on ftruncate which just uses the open file descriptor, the path won't get parsed. It would be interesting to vary the file system that the tmpDir is put on, but I don't think we have much visibility or control over that...

[rootbeer]

Oops, on MacOS it looks like you're allowed to create a 1PB file. Neat! I guess I'll make the test agnostic to this passing or failing ...

error: 'fs.test.test.setEndPos' failed: expected error.FileTooBig, found void
/Users/ci/zach2/_work/zig/zig/lib/std/testing.zig:50:9: 0x10320998f in expectError__anon_752262 (test)
        return error.TestExpectedError;
        ^
/Users/ci/zach2/_work/zig/zig/lib/std/fs/test.zig:1[438](https://github.com/ziglang/zig/actions/runs/13876305970/job/38829056211?pr=23260#step:3:439):5: 0x10320a9e7 in test.setEndPos (test)
    try testing.expectError(error.FileTooBig, f.setEndPos(0x4_0000_0000_0000));
    ^

[squeek502]

the path of the file isn't relevant.

Ah, true, it'd only be exercising Dir.openFile, nevermind then