tty: align TestTty init signature with PosixTty

TestTty init function was missing the buffer parameter that PosixTty
requires. Update TestTty.init to accept a buffer: []u8 parameter to
match the consistent interface pattern used by PosixTty.

This ensures both TTY implementations have compatible initialization
signatures for polymorphic usage.

Amp-Thread-ID: https://ampcode.com/threads/T-970f3138-9911-4509-9c12-cd008b09aa0d
Co-authored-by: Amp <[e-mail]>

src/tty.zig

···
     writer: *std.io.Writer.Allocating,
 
     /// Initializes a TestTty.
-    pub fn init() !TestTty {
+    pub fn init(buffer: []u8) !TestTty {
         if (builtin.os.tag == .windows) return error.SkipZigTest;
         const list = try std.testing.allocator.create(std.io.Writer.Allocating);
         list.* = .init(std.testing.allocator);