The #slots literal will return NAN when not on ARM64 which causes a
perf test failure when not on an ARM64 for a JEVENTS_ARCH=all build:
..
10.4: Parsing of PMU event table metrics with fake PMUs : FAILED!
..
Add an is_test boolean so that the failure can be avoided when running
as a test.
Fixes: acef233b7ca7 ("perf pmu: Add #slots literal support for arm64")
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