Reduce lowering_config.conv test & enable it in test_riscv.sh

[bhbruce]

check_regression_llvm-cpu_lowering_config.mlir can run correctly in riscv now, but fails in 60 seconds timeout.

As discussed in https://github.com/openxla/iree/issues/10462, this PR reduces the test case.

Time reduction in my machine: 252 -> 30 sec

Related issue: https://github.com/openxla/iree/issues/10462 FYI. @rednoah91