shared/tinyusb: Only run TinyUSB on the main thread if GIL is disabled.

If GIL is disabled then there's threat of a race condition if some other
code specifically requests USB processing (i.e. to unblock stdio), while
a scheduled TinyUSB callback is already running on another thread.

Relies on the change in the parent commit, where scheduler is restricted
to main thread if GIL is disabled.

Fixes #15390 - "TinyUSB callback can't recurse" exceptions on rp2 when
using _thread module and USB serial I/O.

Adds a unit test for stdin functioning correctly in threads (fails on rp2
port without this fix).

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

shared/tinyusb/mp_usbd_runtime.c

@@ -501,6 +501,15 @@ void mp_usbd_task_callback(mp_sched_node_t *node) {
 // Task function can be called manually to force processing of USB events
 // (mostly from USB-CDC serial port when blocking.)
 void mp_usbd_task(void) {
+    #if MICROPY_PY_THREAD && !MICROPY_PY_THREAD_GIL
+    if (!mp_thread_is_main_thread()) {
+        // Avoid race with the scheduler callback by scheduling TinyUSB to run
+        // on the main thread.
+        mp_usbd_schedule_task();
+        return;
+    }
+    #endif
+
     if (in_usbd_task) {
         // If this exception triggers, it means a USB callback tried to do
         // something that itself became blocked on TinyUSB (most likely: read or