Tests: Fix MacOS test driver when child process finishes (#6032) (#6038)

os.read(fd, 1) returns b'' on MacOS when the
child process exits, whereas it must be throwing an OSError
on other platforms.

test_regress/driver.py

@@ -1734,6 +1734,9 @@ class VlTest:
                     try:
                         data = os.read(fd, 1)
                         self._run_output(data, logfh, tee)
+                        # Parent detects child termination by checking for b''
+                        if not data:
+                            break
                     except OSError:
                         break