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/transport.h b/fastboot/transport.h
index 67d01f9..96b90d2 100644
--- a/fastboot/transport.h
+++ b/fastboot/transport.h
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-#ifndef TRANSPORT_H_
-#define TRANSPORT_H_
+#pragma once
 
 #include <android-base/macros.h>
 
@@ -44,5 +43,3 @@
   private:
     DISALLOW_COPY_AND_ASSIGN(Transport);
 };
-
-#endif  // TRANSPORT_H_