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/tcp.h b/fastboot/tcp.h
index aa3ef13..8b638a4 100644
--- a/fastboot/tcp.h
+++ b/fastboot/tcp.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef TCP_H_
-#define TCP_H_
+#pragma once
 
 #include <memory>
 #include <string>
@@ -55,5 +54,3 @@
 }  // namespace internal
 
 }  // namespace tcp
-
-#endif  // TCP_H_