test_cell: Test CellTypes::eval() directly

Where possible at least, and disable-able with `-nocteval`.
`$lcu` doesn't work because it uses ports `P, G, CI, CO` instead of the standard `A, B, C, Y`.
`$alu` and `$fa` have more than one output so they don't work either (and in the case of `$alu` it has extra inputs too).
`$macc` is at least supported, but `CellTypes::eval()` doesn't have an implementation so it fails (which would also be true for `$lcu`, `$alu`, and `$fa`; if they weren't being rejected based on ports).

Also add `test_cell -list [all|evaluable|missing]` which prints the list of cell types supported by test_cell, cell types marked evaluable, and cell types marked evaluable but not supported by test_cell respectively.  Potential for listing cell types supported by test_cell but *not* marked evalulable, though that list is currently empty.

Add `tests/various/evaluable.sh` to exercise this.

tests/various/evaluable.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+yosys=../../yosys
+log="evalcells.log"
+
+bad=
+
+$yosys -QTL $log -qp 'test_cell -list all'
+while read line;
+do
+    if [[ "$line" =~ ^\$.* ]]; then
+        if ! $yosys -qqp test_cell\ -n\ 1\ -s\ 1\ "$line"; then
+            bad=1
+        fi
+    fi
+done <$log
+
+if [ $bad ]; then
+    echo 'One or more evaluable cells failed testing.'
+    exit 1
+else
+    echo 'All evaluable cells match behavior.'
+fi