VCS X bugs are context-fragile: test one case per file

When characterizing a simulator 4-state bug (e.g. VCS {N{x}}==const returning 0), never put several test cases in one module: adding an unused wire w = {4{en}}; or a procedural a = {4{en}}; before the compare made VCS return the correct x, so combined testbenches hide the bug and readers' copy-paste repros fail. Generate one isolated file per (expression, x-source, surrounding code, sink, dump mode) and keep the exact minimal file as the public repro. Also, Icarus 12 $isunknown on an expression can return 1 when the value is 0 under $dumpvars, so don't use $isunknown as a referee sink.