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/protocol.cpp b/fastboot/protocol.cpp
index 133a2d0..8c20262 100644
--- a/fastboot/protocol.cpp
+++ b/fastboot/protocol.cpp
@@ -57,10 +57,10 @@
 }
 
 static int64_t check_response(Transport* transport, uint32_t size, char* response) {
-    char status[65];
+    char status[FB_RESPONSE_SZ + 1];
 
     while (true) {
-        int r = transport->Read(status, 64);
+        int r = transport->Read(status, FB_RESPONSE_SZ);
         if (r < 0) {
             g_error = android::base::StringPrintf("status read failed (%s)", strerror(errno));
             transport->Close();
@@ -120,7 +120,7 @@
 
 static int64_t _command_start(Transport* transport, const std::string& cmd, uint32_t size,
                               char* response) {
-    if (cmd.size() > 64) {
+    if (cmd.size() > FB_COMMAND_SZ) {
         g_error = android::base::StringPrintf("command too large (%zu)", cmd.size());
         return -1;
     }