add tests for cast to and from enum (#6275)

Author: nwt

runtime/ztests/expr/cast/enum.yaml

@@ -0,0 +1,19 @@
+spq: this::enum(e1,e2)
+
+vector: true
+
+input: |
+  "e2"
+  "e3"
+  0
+  null
+  null::string
+  null::int64
+
+output: |
+  "e2"::enum(e1,e2)
+  error({message:"no such symbol in enum(e1,e2)",on:"e3"})
+  error({message:"cannot cast to enum",on:0})
+  null::enum(e1,e2)
+  null::enum(e1,e2)
+  null::enum(e1,e2)

runtime/ztests/expr/cast/string.yaml

@@ -28,6 +28,7 @@ input: |
   {foo:"bar"}
   null::{foo:string}
   [1,2,3]
+  "e2"::enum(e1,e2)
 
 output: |
   "1"
@@ -55,3 +56,4 @@ output: |
   "{foo:\"bar\"}"
   null::string
   "[1,2,3]"
+  "e2"