Introduce upgrade tests from released versions (#2955)

## What does this PR do?

This PR adds upgrade tests for both standalone and managed agent from
some versions of the current and previous major release to the latest
snapshot.

The tests will select:
- up to 2 versions with the same major version as the current agent
- up to 1 version from a previous major version
These versions are retrieved using the Artifact API and working
backwards through the versions, excluding snapshots.
The _up to_ part is because under certain circumstances (new major
release, for example) we may not find enough versions fitting the
criteria.

The tests `TestStandaloneUpgrade` and `TestFleetManagedUpgrade` will
start a subtest (using `t.Run()`) for each version to be used as a
starting point for the upgrade and then upgrade it to the latest version
of agent.

Current limitations:
- We are currently using the ArtifactFetcher so the starting elastic
agent version all come from the artifact API. For all agent versions
that allow the `--skip-verify` we use the packaged elastic agent as
artifact for the upgrade via `--sourceURI` parameter

## Why is it important?
We need to have automated tests for the common upgrade scenarios to
detect upgrade issues as soon as possible in the development cycle.

## How to test this PR locally

We can run the integration tests using
`AGENT_VERSION="8.10.0-SNAPSHOT" GOTEST_FLAGS="-test.run
^(TestFleetManagedUpgrade|TestStandaloneUpgrade)$" mage
integration:test`
or if you want to run just a specific version upgrade scenario we can
use (for example only standalone upgrade from 8.9.0 to 8.10.0-SNAPSHOT):
`AGENT_VERSION="8.10.0-SNAPSHOT" GOTEST_FLAGS="-test.run
^TestStandaloneUpgrade/Upgrade_8.9.0_to_8.10.0-SNAPSHOT$"`

---------

Co-authored-by: Blake Rouse <[email protected]>
(cherry picked from commit cfd059d)

# Conflicts:
#	testing/integration/upgrade_test.go

pkg/testing/fixture.go

@@ -42,7 +42,8 @@ type Fixture struct {
 	allowErrs       bool
 	connectTimout   time.Duration
 
-	workDir string
+	srcPackage string
+	workDir    string
 
 	installed   bool
 	installOpts *InstallOpts
@@ -153,6 +154,7 @@ func (f *Fixture) Prepare(ctx context.Context, components ...UsableComponent) er
 	if err != nil {
 		return err
 	}
+	f.srcPackage = src
 	filename := filepath.Base(src)
 	name, _, err := splitFileType(filename)
 	if err != nil {
@@ -191,6 +193,15 @@ func (f *Fixture) WorkDir() string {
 	return f.workDir
 }
 
+// SrcPackage returns the location on disk of the elastic agent package used by this fixture.
+func (f *Fixture) SrcPackage(ctx context.Context) (string, error) {
+	err := f.ensurePrepared(ctx)
+	if err != nil {
+		return "", err
+	}
+	return f.srcPackage, nil
+}
+
 func ExtractArtifact(l Logger, artifactFile, outputDir string) error {
 	filename := filepath.Base(artifactFile)
 	_, ext, err := splitFileType(filename)