Explain test build tags and env variables

Author: stephanos

docs/development/testing.md

@@ -2,6 +2,17 @@
 
 This document describes the project's testing utilities and best practices.
 
+## Setup
+
+### Build tags
+- `test_dep` (required): This Go build tag is required for running functional tests.
+
+### Environment variables
+- `CGO_ENABLED`: Set to 0 to disable CGO, which can significantly speed up compilation time.
+- `TEMPORAL_DEBUG`: Extends functional test timeouts to allow sufficient time for debugging sessions.
+- `TEMPORAL_TEST_LOG_FORMAT`: Controls the output format for test logs. Available options: `json` or `console`
+- `TEMPORAL_TEST_LOG_LEVEL`:  Sets the verbosity level for test logging. Available levels: `debug`, `info`, `warn`, `error`, `fatal`
+
 ## Test helpers
 
 Test helpers can be found in the [common/testing](../../common/testing) package.