compiletest: try running lldb_batchmode.py with PYTHONUNBUFFERED

When reporting fatal errors, LLVM calls abort() to exit the program.
There is a chance that might interfere with Python printing stuff to
stdout, as by default it relies on buffering to increase performance.

This commit tries to disable Python buffering, to hopefully get useful
logs while debugging #78665.

src/tools/compiletest/src/runtest.rs

@@ -1166,6 +1166,7 @@ impl<'test> TestCx<'test> {
                 .arg(&lldb_script_path)
                 .arg(test_executable)
                 .arg(debugger_script)
+                .env("PYTHONUNBUFFERED", "1") // Help debugging #78665
                 .env("PYTHONPATH", self.config.lldb_python_dir.as_ref().unwrap()),
         )
     }