sailcov test: add test for nested mapping with guard

This test is reduced from a problem encountered in the RISCV model
(see #639).  Currently fails due sailcov producing an assertion
failure warning.

test/sailcov/nested_mapping.sail

@@ -0,0 +1,22 @@
+default Order dec
+$include <prelude.sail>
+
+union ast = {
+  B : (bool),
+  Z : unit
+}
+
+mapping bool_not_bits : bool <-> bits(1) = {
+  true   <-> 0b0,
+  false  <-> 0b1
+}
+
+mapping encdec  : bits(2) <->  ast = {
+  0b1 @ bool_not_bits(s) if true <-> B(s),
+  0b00 <-> Z()
+}
+
+val main : unit -> unit
+function main() = {
+    let _ = encdec(0b00)
+}