contracts, tests: fix global variable modifications and remove unsafe parallelism (#1882)

Add defer cleanup for global variables and remove t.Parallel() where
global state is modified to prevent race conditions and test pollution.

Changes:

1. tests/vm_test.go:
   - Add defer to restore common.TIPXDCXCancellationFee
   - Remove t.Parallel() since modifying global variable makes concurrent
     execution unsafe, even with defer cleanup
   - Prevents potential race conditions with other tests in the package

2. contracts/tests/Inherited_test.go:
   - Add defer to restore common.TIPXDCXCancellationFee
   - No t.Parallel() present, so safe with defer alone

3. contracts/trc21issuer/trc21issuer_test.go:
   - Add defer to restore common.TRC21IssuerSMC (was missing)
   - Fix existing bug: restore correct variable common.TRC21GasPriceBefore
     instead of common.TIPTRC21Fee
   - Remove unused variables: token, delay

Rationale for removing t.Parallel():
While defer ensures cleanup after test completion, during test execution
the modified global variable is visible to all concurrent tests. Even
though tests/vm_test.go is currently the only test modifying
TIPXDCXCancellationFee, removing t.Parallel() is the safer approach to
ensure complete test isolation and prevent timing-dependent behavior.

This follows the same principles as the txpool MinGasPrice fix: global
variable modifications should not occur during concurrent test execution.

contracts/tests/Inherited_test.go

@@ -25,7 +25,12 @@ func TestPriceFeed(t *testing.T) {
 	glogger.Verbosity(log.LevelTrace)
 	log.SetDefault(log.NewLogger(glogger))
 
+	oldTIPXDCXCancellationFee := new(big.Int).Set(common.TIPXDCXCancellationFee)
+	defer func() {
+		common.TIPXDCXCancellationFee = oldTIPXDCXCancellationFee
+	}()
 	common.TIPXDCXCancellationFee = big.NewInt(0)
+
 	// init genesis
 	contractBackend := backends.NewXDCSimulatedBackend(
 		types.GenesisAlloc{

contracts/trc21issuer/trc21issuer_test.go

@@ -23,16 +23,13 @@ var (
 	subKey, _ = crypto.HexToECDSA("5bb98c5f937d176aa399ea6e6541f4db8f8db5a4ee1a8b56fb8beb41f2d755e3")
 	subAddr   = crypto.PubkeyToAddress(subKey.PublicKey) //0x21292d56E2a8De3cC4672dB039AAA27f9190B1f6
 
-	token = common.HexToAddress("0000000000000000000000000000000000000089")
-
-	delay    = big.NewInt(30 * 48)
 	minApply = big.NewInt(0).Mul(big.NewInt(1000), big.NewInt(100000000000000000)) // 100 XDC
 )
 
 func TestFeeTxWithTRC21Token(t *testing.T) {
-	oldTRC21GasPriceBefore := new(big.Int).Set(common.TIPTRC21Fee)
+	oldTRC21GasPriceBefore := new(big.Int).Set(common.TRC21GasPriceBefore)
 	defer func() {
-		common.TIPTRC21Fee = oldTRC21GasPriceBefore
+		common.TRC21GasPriceBefore = oldTRC21GasPriceBefore
 	}()
 	common.TRC21GasPriceBefore = big.NewInt(1)
 
@@ -54,7 +51,12 @@ func TestFeeTxWithTRC21Token(t *testing.T) {
 	contractBackend.Commit()
 
 	// set contract address to config
+	oldTRC21IssuerSMC := common.TRC21IssuerSMC
+	defer func() {
+		common.TRC21IssuerSMC = oldTRC21IssuerSMC
+	}()
 	common.TRC21IssuerSMC = trc21IssuerAddr
+
 	cap := big.NewInt(0).Mul(big.NewInt(10000000), big.NewInt(10000000000000))
 	TRC21fee := big.NewInt(100)
 

tests/vm_test.go

@@ -25,8 +25,11 @@ import (
 )
 
 func TestVM(t *testing.T) {
+	oldTIPXDCXCancellationFee := new(big.Int).Set(common.TIPXDCXCancellationFee)
+	defer func() {
+		common.TIPXDCXCancellationFee = oldTIPXDCXCancellationFee
+	}()
 	common.TIPXDCXCancellationFee = big.NewInt(100000000)
-	t.Parallel()
 	vmt := new(testMatcher)
 	vmt.fails("^vmSystemOperationsTest.json/createNameRegistrator$", "fails without parallel execution")