unit tests: Fix retrying with a new baud rate in IDFDUT.start_app()

tools/tiny-test-fw/IDF/IDFDUT.py

@@ -104,13 +104,14 @@ class IDFDUT(DUT.SerialDUT):
         return cls.get_mac(app, port) is not None
 
     @_uses_esptool
-    def start_app(self, esp, erase_nvs=ERASE_NVS):
+    def _try_flash(self, esp, erase_nvs, baud_rate):
         """
-        download and start app.
+        Called by start_app() to try flashing at a particular baud rate.
 
-        :param: erase_nvs: whether erase NVS partition during flash
+        Structured this way so @_uses_esptool will reconnect each time
-        :return: None
         """
+        try:
+            # note: opening here prevents us from having to seek back to 0 each time
             flash_files = [ (offs, open(path, "rb")) for (offs, path) in self.app.flash_files ]
 
             if erase_nvs:
@@ -144,19 +145,27 @@ class IDFDUT(DUT.SerialDUT):
                 False
             )
 
-        try:
-            for baud_rate in [ 921600, 115200 ]:
-                try:
             esp.change_baud(baud_rate)
             esptool.write_flash(esp, flash_args)
+        finally:
+            for (_,f) in flash_files:
+                f.close()
+
+    def start_app(self, erase_nvs=ERASE_NVS):
+        """
+        download and start app.
+
+        :param: erase_nvs: whether erase NVS partition during flash
+        :return: None
+        """
+        for baud_rate in [ 921600, 115200 ]:
+            try:
+                self._try_flash(erase_nvs, baud_rate)
                 break
             except RuntimeError:
                 continue
         else:
             raise IDFToolError()
-        finally:
-            for (_,f) in flash_files:
-                f.close()
 
     @_uses_esptool
     def reset(self, esp):