gh-109960: Remove test_pty timeout of 10 seconds (#110058)

In 2003, test_pty got a hardcoded timeout of 10 seconds to prevent
hanging on AIX & HPUX "if run after test_openpty":
commit 7d8145268ee282f14d6adce9305dc3c1c7ffec14. Since 2003, test_pty
was no longer reported to hang on AIX. But today, the test can fail
simply because a CI is busy running other tests in parallel.
The timeout of 10 seconds is no longer needed, just remove it.
Moreover, regrtest now has multiple built-in generic timeout
mecanisms.