Fix vmexec PTY output truncation on Linux

[Hokila]

Summary

Fixes #844.

When vmexec configures standard I/O through a pseudo-terminal, the slave terminal was left in cooked mode, which could truncate or otherwise alter larger output streams. This change configures the slave PTY in raw mode before attaching it to stdin/stdout/stderr.

Changes

• Configure the PTY slave with Terminal.setraw() in Console.configureStdIO().
• Add the vmexecTests test target.
• Add a Linux regression test that writes and verifies 4096 bytes through Console.configureStdIO().
• Keep the test compatible with Swift 6.3 Linux builds.

Validation

• make linux-test
  • 656 tests passed across 86 suites.
• Focused vmexecTests
  • 1 test passed.
• Swift formatting, license, and whitespace checks passed.