fix: spansql: fix invalid CAST tests

A misreading of the spanner docs lead to tests that indicated that
casting `AS ENUM` or `AS PROTO` was valid syntax (despite not specifying
_which_ protobuf enum or message type to cast to). Replace these cases
with ones that validate casting to specific enum/message types.

Thanks to @apstndb for calling this out on googleapis#10945.

spanner/spansql/parser_test.go

@@ -444,8 +444,8 @@ func TestParseExpr(t *testing.T) {
 		// Functions
 		{`STARTS_WITH(Bar, 'B')`, Func{Name: "STARTS_WITH", Args: []Expr{ID("Bar"), StringLiteral("B")}}},
 		{`CAST(Bar AS STRING)`, Func{Name: "CAST", Args: []Expr{TypedExpr{Expr: ID("Bar"), Type: Type{Base: String}}}}},
-		{`CAST(Bar AS ENUM)`, Func{Name: "CAST", Args: []Expr{TypedExpr{Expr: ID("Bar"), Type: Type{Base: Enum}}}}},
-		{`CAST(Bar AS PROTO)`, Func{Name: "CAST", Args: []Expr{TypedExpr{Expr: ID("Bar"), Type: Type{Base: Proto}}}}},
+		{`CAST(Bar AS fizzle.bit)`, Func{Name: "CAST", Args: []Expr{TypedExpr{Expr: ID("Bar"), Type: Type{Base: Proto, ProtoRef: "fizzle.bit"}}}}},
+		{`CAST(Bar AS fizzle.bit.baz)`, Func{Name: "CAST", Args: []Expr{TypedExpr{Expr: ID("Bar"), Type: Type{Base: Proto, ProtoRef: "fizzle.bit.baz"}}}}},
 		{`SAFE_CAST(Bar AS INT64)`, Func{Name: "SAFE_CAST", Args: []Expr{TypedExpr{Expr: ID("Bar"), Type: Type{Base: Int64}}}}},
 		{`EXTRACT(DATE FROM TIMESTAMP AT TIME ZONE "America/Los_Angeles")`, Func{Name: "EXTRACT", Args: []Expr{ExtractExpr{Part: "DATE", Type: Type{Base: Date}, Expr: AtTimeZoneExpr{Expr: ID("TIMESTAMP"), Zone: "America/Los_Angeles", Type: Type{Base: Timestamp}}}}}},
 		{`EXTRACT(DAY FROM DATE)`, Func{Name: "EXTRACT", Args: []Expr{ExtractExpr{Part: "DAY", Expr: ID("DATE"), Type: Type{Base: Int64}}}}},