Remove K6_BROWSER_HEADLESS=true

Since the k6 browser module defaults to headless true already, there's
no need to be explicit about this, so we've removed it from the ci test
workflow.

.github/workflows/test.yml

@@ -46,7 +46,7 @@ jobs:
             args[1]="1"
             export GOMAXPROCS=1
           fi
-          K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome K6_BROWSER_HEADLESS=true go test "${args[@]}" -timeout 5m ./...
+          K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome go test "${args[@]}" -timeout 5m ./...
 
   test-tip:
     runs-on: ubuntu-latest
@@ -74,7 +74,7 @@ jobs:
           go version
           export GOMAXPROCS=2
           args=("-p" "2" "-race")
-          K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome K6_BROWSER_HEADLESS=true go test "${args[@]}" -timeout 5m ./...
+          K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome go test "${args[@]}" -timeout 5m ./...
 
   test-current-cov:
     strategy:
@@ -109,7 +109,7 @@ jobs:
                   list=$(echo "$list" | cut -f1 -d ' ' | sort -u | paste -sd, -)
               fi
 
-              K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome K6_BROWSER_HEADLESS=true go test "${args[@]}" -timeout 5m --coverpkg="$list" -coverprofile=$(echo $pkg | tr / -).coverage $pkg
+              K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome go test "${args[@]}" -timeout 5m --coverpkg="$list" -coverprofile=$(echo $pkg | tr / -).coverage $pkg
           done
           grep -h -v "^mode:" *.coverage >> coverage.txt
           rm -f *.coverage
@@ -153,4 +153,4 @@ jobs:
           go get go.k6.io/k6@master
           go mod tidy
           cat go.mod | grep go.k6.io/k6
-          K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome K6_BROWSER_HEADLESS=true go test "${args[@]}" -timeout 5m ./...
+          K6_BROWSER_EXECUTABLE_PATH=/usr/bin/google-chrome go test "${args[@]}" -timeout 5m ./...