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/socket_mock.h b/fastboot/socket_mock.h
index eacd6bb..6e95b16 100644
--- a/fastboot/socket_mock.h
+++ b/fastboot/socket_mock.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef SOCKET_MOCK_H_
-#define SOCKET_MOCK_H_
+#pragma once
 
 #include <memory>
 #include <queue>
@@ -97,5 +96,3 @@
 
     DISALLOW_COPY_AND_ASSIGN(SocketMock);
 };
-
-#endif  // SOCKET_MOCK_H_