added caching mechanism #1464

Job	Run time
unittest	1m 40s
collect-experiments	3s
run-test (tests/ci/rand_soc.yaml)	33m 2s
run-test (tests/ci/randsoc_dumped.yaml)	39m 11s
run-test (tests/ci/vivado_bit_analysis.yaml)	12m 50s
run-test (tests/ci/vivado_bit_to_netlist.yaml)	10m 21s
run-test (tests/ci/vivado_conformal.yaml)	3m 11s
run-test (tests/ci/vivado_ooc.yaml)	13m 40s
run-test (tests/ci/vivado_phys_netlist.yaml)	19m 59s
run-test (tests/ci/vivado_phys_netlist_cmp.yaml)	14m 4s
run-test (tests/ci/vivado_phys_netlist_error_inject.yaml)	26m 39s
run-test (tests/ci/vivado_wafove.yaml)	21m 13s
run-test (tests/ci/vivado_yosys_impl.yaml)	8m 23s
	3h 24m 16s

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