Add test for accessing const from namespace

test/contracts/toplevel_constants.aes

@@ -1,3 +1,9 @@
+namespace N0 =
+    let nsconst = 1
+
+namespace N =
+    let nsconst = N0.nsconst
+
 contract C =
     datatype event = EventX(int, string)
 
@@ -25,6 +31,7 @@ contract C =
     let c19 = ok_2YNyxd6TRJPNrTcEDCe9ra59SVUdp9FR9qWC5msKZWYD9bP9z5
     let c20 = oq_2oRvyowJuJnEkxy58Ckkw77XfWJrmRgmGaLzhdqb67SKEL1gPY
     let c21 = ct_Ez6MyeTMm17YnTnDdHTSrzMEBKmy7Uz2sXu347bTDPgVH2ifJ : C
+    let c22 = N.nsconst
 
     entrypoint f01() = c01
     entrypoint f02() = c02
@@ -47,3 +54,4 @@ contract C =
     entrypoint f19() = c19
     entrypoint f20() = c20
     entrypoint f21() = c21
+    entrypoint f22() = c22