Convert per-i915-engine subtest enumeration to use dynamic subtests instead

• gem_busy
• gem_cs_tlb
• gem_ctx_exec
• gem_ctx_isolation
• gem_ctx_persistence
• gem_ctx_switch
• gem_exec_basic
• gem_exec_parallel
• gem_exec_store
• gem_spin_batch
• gem_wait
• i915_hangman
• perf_pmu