[Elastic Agent] improve CI reporting (elastic#20780)

The elastic agent used to be in his own repository and didn't have
access to the beats toolchains and setup like testings. This PR move the
custom mage test:unit implement to use the devtools' implementations.

This changes will allow CI to correctly generates reports for failures.

Fixes: elastic#19822

x-pack/elastic-agent/magefile.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"os"
@@ -236,13 +237,10 @@ func (Test) All() {
 }
 
 // Unit runs all the unit tests.
-func (Test) Unit() error {
+func (Test) Unit(ctx context.Context) error {
 	mg.Deps(Prepare.Env, Build.TestBinaries)
-	raceFlag := ""
-	if os.Getenv("DEV_ARCH") == "amd64" {
-		raceFlag = "-race"
-	}
-	return RunGo("test", raceFlag, "-v", "-coverprofile", filepath.Join(buildDir, "coverage.out"), "./...")
+	params := devtools.DefaultGoTestUnitArgs()
+	return devtools.GoTest(ctx, params)
 }
 
 // Coverage takes the coverages report from running all the tests and display the results in the browser.