Add fastboot_test.cpp and test --os-version/--os-patch-level parsing.

Also switch all remaining headers over to #pragma once, and actually
use FB_COMMAND_SZ.

Bug: http://b/77340848
Test: ran tests
Change-Id: I27107d054c206e66c39208099e36a55df604e08f
diff --git a/fastboot/usb.h b/fastboot/usb.h
index 4acf12d..5b44468 100644
--- a/fastboot/usb.h
+++ b/fastboot/usb.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef _USB_H_
-#define _USB_H_
+#pragma once
 
 #include "transport.h"
 
@@ -56,5 +55,3 @@
 typedef int (*ifc_match_func)(usb_ifc_info *ifc);
 
 Transport* usb_open(ifc_match_func callback);
-
-#endif