try to fix flaky mysql test

Author: wxiaoguang

models/db/collation.go

@@ -68,7 +68,8 @@ func CheckCollations(x *xorm.Engine) (*CheckCollationsResult, error) {
 
 	var candidateCollations []string
 	if x.Dialect().URI().DBType == schemas.MYSQL {
-		if _, err = x.SQL("SELECT @@collation_database").Get(&res.DatabaseCollation); err != nil {
+		_, err = x.SQL("SELECT DEFAULT_COLLATION_NAME FROM INFORMATION_SCHEMA.SCHEMATA WHERE SCHEMA_NAME = ?", setting.Database.Name).Get(&res.DatabaseCollation)
+		if err != nil {
 			return nil, err
 		}
 		res.IsCollationCaseSensitive = func(s string) bool {

tests/integration/db_collation_test.go

@@ -73,9 +73,12 @@ func TestDatabaseCollation(t *testing.T) {
 
 	t.Run("Convert tables to utf8mb4_bin", func(t *testing.T) {
 		defer test.MockVariableValue(&setting.Database.CharsetCollation, "utf8mb4_bin")()
-		assert.NoError(t, db.ConvertDatabaseTable())
 		r, err := db.CheckCollations(x)
 		assert.NoError(t, err)
+		assert.EqualValues(t, "utf8mb4_bin", r.ExpectedCollation)
+		assert.NoError(t, db.ConvertDatabaseTable())
+		r, err = db.CheckCollations(x)
+		assert.NoError(t, err)
 		assert.Equal(t, "utf8mb4_bin", r.DatabaseCollation)
 		assert.True(t, r.CollationEquals(r.ExpectedCollation, r.DatabaseCollation))
 		assert.Empty(t, r.InconsistentCollationColumns)