use DisableGPU in the tests

On my i5-8350U, the option takes 'go test' from ~11s to ~10s, from a
manual look over a few runs.

chromedp_test.go

@@ -17,6 +17,7 @@ var (
 		NoFirstRun,
 		NoDefaultBrowserCheck,
 		Headless,
+		DisableGPU,
 	}
 )
 
@@ -59,10 +60,6 @@ func TestMain(m *testing.M) {
 	if noSandbox := os.Getenv("CHROMEDP_NO_SANDBOX"); noSandbox != "false" {
 		allocOpts = append(allocOpts, NoSandbox)
 	}
-	// must be explicitly set, as disabling gpu slows unit tests
-	if disableGPU := os.Getenv("CHROMEDP_DISABLE_GPU"); disableGPU != "" && disableGPU != "false" {
-		allocOpts = append(allocOpts, DisableGPU)
-	}
 
 	ctx, cancel := NewAllocator(context.Background(), WithExecAllocator(allocOpts...))
 	allocCtx = ctx