Revert "internal/processtest: prevent parallel process testing"

This reverts commit 397da70815.

Reason: This did not resolve the issue: https://github.com/hajimehoshi/ebiten/actions/runs/4180959132/jobs/7242421909

Updates #2571

internal/processtest/processtest_test.go

@@ -42,11 +42,7 @@ func TestPrograms(t *testing.T) {
 			continue
 		}
 
-		// Run tests not in parallel (#2571)
-		ch := make(chan struct{})
 		t.Run(n, func(t *testing.T) {
-			defer close(ch)
-
 			cmd := exec.Command("go", "run", filepath.Join(dir, n))
 			stderr := &bytes.Buffer{}
 			cmd.Stderr = stderr
@@ -54,6 +50,5 @@ func TestPrograms(t *testing.T) {
 				t.Errorf("%v\n%s", err, stderr)
 			}
 		})
-		<-ch
 	}
 }