Add XK6_HEADLESS to the CI test workflow

This is in preparation for ensuring that the
TestFrameNoPanicWithEmbeddedIFrame isn't ran in CI, but it will run
where that env var isn't set, allowing us to ensure we don't break
the fix.

Resolves: #677 (comment)

Author: ankur22

.github/workflows/test.yml

@@ -43,6 +43,7 @@ jobs:
             args[1]="1"
             export GOMAXPROCS=1
           fi
+          export XK6_HEADLESS=true
           go test "${args[@]}" -timeout 5m ./...
 
   test-tip:
@@ -71,6 +72,7 @@ jobs:
           go version
           export GOMAXPROCS=2
           args=("-p" "2" "-race")
+          export XK6_HEADLESS=true
           go test "${args[@]}" -timeout 5m ./...
 
   test-current-cov:
@@ -99,6 +101,7 @@ jobs:
             args[1]="1"
             export GOMAXPROCS=1
           fi
+          export XK6_HEADLESS=true
           echo "mode: set" > coverage.txt
           for pkg in $(go list ./... | grep -v vendor); do
               list=$(go list -test -f  '{{ join .Deps  "\n"}}' $pkg | grep github.com/grafana/xk6-browser | grep -v vendor || true)