createTempDir register a test cleanup function to remove the folder it
created, however, on Windows, this folder sometimes fails to be
removed because there are still open file handlers for the files
within the folder.

We fix this problem calling install.RemovePath that will retry removing
the folder for about 2s. This is a very similar approach to
what Go's t.TempDir does.

Fix the flakiness from TestUpgradeHandler* tests by re-working the
mockUpgradeManager, now it accepts a function for its Upgrade method
and their implementation is goroutine safe

(cherry picked from commit 1242e71)

# Conflicts:
#	internal/pkg/agent/application/actions/handlers/handler_action_upgrade_test.go
#	pkg/testing/fixture.go
#	testing/integration/event_logging_test.go

createTempDir register a test cleanup function to remove the folder it
created, however, on Windows, this folder sometimes fails to be
removed because there are still open file handlers for the files
within the folder.

We fix this problem calling install.RemovePath that will retry removing
the folder for about 2s. This is a very similar approach to
what Go's t.TempDir does.

Fix the flakiness from TestUpgradeHandler* tests by re-working the
mockUpgradeManager, now it accepts a function for its Upgrade method
and their implementation is goroutine safe

(cherry picked from commit 1242e71)

TestLogIngestionFleetManaged was failing because the namespace generated by the integration tests framework was not unique among different tests and test runs, so sometimes collisions would occurs causing some tests to be flaky.

TestDebLogIngestFleetManaged was failing because it also has got Beats logging connection errors before receiving the configuration from Elastic-Agent, now this message is also in the allow list.

When testing .deb the AGENT_KEEP_INSTALLED environment variable is respected.

When an integration test fails, the work directory created by the framework is now kept and its path is printed.

createTempDir register a test cleanup function to remove the folder it created, however, on Windows, this folder sometimes fails to be removed because there are still open file handlers for the files within the folder.

We fix this problem by retrying to remove the folder with a maximum overall wait time of 3 seconds. This is a very similar approach to what Go's t.TempDir does.

Fix the flakiness from TestUpgradeHandler* tests by re-working the
mockUpgradeManager, now it accepts a function for its Upgrade method
and their implementation is goroutine safe

TestEnvWithDefault
Now TestEnvWithDefault unsets all environment variables it sets,
allowing it to be run multiple times using -count.

TestContainerCMDEventToStderr
TestContainerCMDEventToStderr did not call agentFixture.Prepare early
enough leading to an empty STATE_PATH env var, so all state
information was in /usr/share/elastic-agent, which could make
subsequent tests to fail because they could read
/usr/share/elastic-agent/state/container-paths.yml and use a state
path different than the one set in the test.