kernel32/tests: use ret in load_blackbox()

[Austin English]

Passes WineTestBot:
https://testbot.winehq.org/JobDetails.pl?Key=2240

-- 
-Austin
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diff --git a/dlls/kernel32/tests/debugger.c b/dlls/kernel32/tests/debugger.c
index e116c9d..cd6dbc5 100644
--- a/dlls/kernel32/tests/debugger.c
+++ b/dlls/kernel32/tests/debugger.c
@@ -144,6 +144,7 @@ static int load_blackbox(const char* logfile, void* blackbox, int size)
         return 0;
     }
     ret=ReadFile(hFile, blackbox, size, &read, NULL);
+    ok(ret, "ReadFile failed: %08x\n", GetLastError());
     ok(read == size, "wrong size for '%s': read=%d\n", logfile, read);
     CloseHandle(hFile);
     return 1;