arb_shader_clock: fix tests
The counter exposed by this ARB_shader_clock is "monotonically incrementing [...] within a single shader invocation" so we can't compare the value from one dispatch with one from a later dispatch.
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The counter exposed by this ARB_shader_clock is "monotonically incrementing [...] within a single shader invocation" so we can't compare the value from one dispatch with one from a later dispatch.