Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

models/db/engine_test.go

@@ -30,7 +30,7 @@ func TestDumpDatabase(t *testing.T) {
 	assert.NoError(t, db.GetEngine(t.Context()).Sync(new(Version)))
 
 	for _, dbType := range setting.SupportedDatabaseTypes {
-		assert.NoError(t, db.DumpDatabase(filepath.Join(dir, dbType+".sql"), dbType))
+		assert.NoError(t, db.DumpDatabase(filepath.Join(dir, dbType+".sql"), setting.DatabaseType(dbType)))
 	}
 }