tests: Run async2sync before sat and/or sim to handle $check cells

Right now neither `sat` nor `sim` have support for the `$check` cell.
For formal verification it is a good idea to always run either
async2sync or clk2fflogic which will (in a future commit) lower `$check`
to `$assert`, etc.

While `sim` should eventually support `$check` directly, using
`async2sync` is ok for the current tests that use `sim`, so this commit
also runs `async2sync` before running sim on designs containing
assertions.

tests/verilog/asgn_expr.ys

@@ -1,3 +1,4 @@
 read_verilog -sv asgn_expr.sv
 proc
+async2sync
 sat -verify -prove-asserts -show-all