mage: clean up .agent-testing folder on mage clean (#8123) (#8133)

(cherry picked from commit df5b630)

Co-authored-by: Mauri de Souza Meneguzzo <[email protected]>

Author: mergify[bot]

dev-tools/mage/clean.go

@@ -12,6 +12,7 @@ import (
 // The values may contain variables and will be expanded at the time of use.
 var DefaultCleanPaths = []string{
 	"build",
+	".agent-testing",
 	"docker-compose.yml.lock",
 	"{{.BeatName}}",
 	"{{.BeatName}}.exe",

docs/test-framework-dev-guide.md

@@ -436,6 +436,10 @@ If you encounter any errors mentioning `ogc`, try running `mage integration:clea
 re-running whatever `mage integration:*` target you were trying to run originally when you
 encountered the error.
 
+### Tests seemingly using a stale Elastic Agent package
+
+If your integration tests seem to be using a stale or outdated version of Elastic Agent, it might be due to a cached copy in the .agent-testing directory. To fix this, run `mage clean` to remove cached artifacts, then rebuild the agent with `mage package`, and finally run the integration tests again.
+
 ### Using a different agent version from the stack version
 
 The agent version is used as a fallback for the stack version to use in integration tests