support pretty printing of StringInterLiteral in asserts too

vlib/v/ast/str.v

@@ -77,6 +77,27 @@ pub fn (x Expr) str() string {
 		StringLiteral {
 			return '"$it.val"'
 		}
+		StringInterLiteral {
+			res := []string
+			res << "'"
+			for i, val in it.vals {
+				res << val
+				if i>=it.exprs.len {
+					continue
+				}
+				res << '$'
+				if it.expr_fmts[i].len > 0 {
+					res << '{'
+					res << it.exprs[i].str()
+					res << it.expr_fmts[i]
+					res << '}'
+				}else{
+					res << it.exprs[i].str()
+				}
+			}
+			res << "'"
+			return res.join('')
+		}
 		BoolLiteral {
 			return it.val.str()
 		}