fix tests: emit .exe binary path on windows

Author: davidahmann

internal/testutil/testutil.go

@@ -20,21 +20,20 @@ func RepoRoot(t *testing.T) string {
 func BuildBinary(t *testing.T, root string) string {
 	t.Helper()
 	bin := filepath.Join(t.TempDir(), "proof")
+	if runtime.GOOS == "windows" {
+		bin += ".exe"
+	}
 	// #nosec G204 -- test helper executes a fixed go build command.
 	cmd := exec.Command("go", "build", "-o", bin, "./cmd/proof")
 	cmd.Dir = root
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("build binary: %v: %s", err, string(out))
 	}
-	if _, err := os.Stat(bin); err == nil {
-		return bin
-	}
-	if _, err := os.Stat(bin + ".exe"); err == nil {
-		return bin + ".exe"
+	if _, err := os.Stat(bin); err != nil {
+		t.Fatalf("build binary output not found at %s: %v", bin, err)
 	}
-	t.Fatalf("build binary output not found at %s (or %s.exe)", bin, bin)
-	return ""
+	return bin
 }
 
 func CommandExitCode(t *testing.T, err error) int {