deqp-runner: Fix prefix handling for Missing testcases.
If you (for example) typoed your binary name for the prefixed variant, you'd get a bunch of errors about duplicate tests instead of a bunch of Missings about how the tests weren't run.
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If you (for example) typoed your binary name for the prefixed variant, you'd get a bunch of errors about duplicate tests instead of a bunch of Missings about how the tests weren't run.