test: fix flaky test_drain_exits_when_deadline_exceeded_before_select

The test was racing - the clock manipulation would either exit too
early (before reading "DEADLINE_TEST") or too late (after reading
"SHOULD_NOT_APPEAR"). Fixed by making timing deterministic: track
when the main output has been flushed, then trigger timeout only
after that point but before reading "SHOULD_NOT_APPEAR".

This fixes intermittent failures where the assertion would fail
because the drain exited at the wrong time.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: jtligon

python/packages/jumpstarter/jumpstarter/exporter/hooks_test.py

@@ -32,6 +32,7 @@ def __init__(self, *, return_drain_data: bool = True) -> None:
         self.parent_fd: int | None = None
         self.eof_seen: bool = False
         self._drain_data_returned: bool = False
+        self._main_output_flushed: bool = False
         self._return_drain_data = return_drain_data
         self._original_openpty = pty.openpty
         self._original_os_read = os.read
@@ -52,8 +53,12 @@ def os_read_with_drain_data(self, fd, size):
                 raise
             if not data:
                 self.eof_seen = True
+                self._main_output_flushed = True
                 return b""
             return data
+        # After EOF, first empty read means main output was flushed
+        if not self._main_output_flushed:
+            self._main_output_flushed = True
         if self._return_drain_data and not self._drain_data_returned:
             self._drain_data_returned = True
             return b"SHOULD_NOT_APPEAR\n"
@@ -83,7 +88,9 @@ def __call__(self) -> float:
         if self._call_count == 1:
             self._deadline = real_time + DRAIN_TIMEOUT_SECONDS
             return real_time
-        if self._call_count == 2:
+        # Stay just before deadline until "DEADLINE_TEST" is read,
+        # then jump past deadline to force early exit before "SHOULD_NOT_APPEAR"
+        if not getattr(self._state, '_main_output_flushed', False):
             return self._deadline - 0.001  # type: ignore[operator]
         return self._deadline + 1.0  # type: ignore[operator]