fix failing plan_test, add more rewriting tests for database()

Signed-off-by: deepthi <deepthi@planetscale.com>

go/vt/sqlparser/expression_rewriting_test.go

@@ -44,6 +44,11 @@ func TestRewrites(in *testing.T) {
 			expected: "SELECT :__vtdbname as `database()`",
 			db:       true, liid: false,
 		},
+		{
+			in:       "SELECT database() from test",
+			expected: "SELECT database() from test",
+			db:       false, liid: false,
+		},
 		{
 			in:       "SELECT last_insert_id() as test",
 			expected: "SELECT :__lastInsertId as test",
@@ -54,6 +59,16 @@ func TestRewrites(in *testing.T) {
 			expected: "SELECT :__lastInsertId + :__vtdbname as `last_insert_id() + database()`",
 			db:       true, liid: true,
 		},
+		{
+			in:       "select (select database()) from test",
+			expected: "select (select database() from dual) from test",
+			db:       false, liid: false,
+		},
+		{
+			in:       "select (select database() from dual) from test",
+			expected: "select (select database() from dual) from test",
+			db:       false, liid: false,
+		},
 		{
 			in:       "select (select database() from dual) from dual",
 			expected: "select (select :__vtdbname as `database()` from dual) as `(select database() from dual)` from dual",

go/vt/vtgate/planbuilder/testdata/filter_cases.txt

@@ -1078,7 +1078,7 @@
       "Name": "user",
       "Sharded": true
     },
-    "Query": "select id from user where :__vtdbname",
+    "Query": "select id from user where database()",
     "FieldQuery": "select id from user where 1 != 1",
     "Table": "user"
   }