MAGETWO-51577: FAT failure on TaxCalculationTest for Downloadable and Configurable product

 -- fixed FAT failures by removing sale rules and tax rules before running tests

Author: Robert He

dev/tests/functional/tests/app/Magento/SalesRule/Test/TestStep/CreateSalesRuleStep.php

@@ -57,9 +57,9 @@ public function __construct(FixtureFactory $fixtureFactory, DeleteAllSalesRuleSt
      */
     public function run()
     {
-        $this->deleteAllSalesRule->run();
         $result['salesRule'] = null;
         if ($this->salesRule !== null) {
+            $this->deleteAllSalesRule->run();
             $salesRule = $this->fixtureFactory->createByCode(
                 'salesRule',
                 ['dataset' => $this->salesRule]

dev/tests/functional/tests/app/Magento/Tax/Test/TestStep/CreateTaxRuleStep.php

@@ -57,9 +57,9 @@ public function __construct(FixtureFactory $fixtureFactory, DeleteAllTaxRulesSte
      */
     public function run()
     {
-        $this->deleteAllTaxRule->run();
         $result['taxRule'] = null;
         if ($this->taxRule !== null) {
+            $this->deleteAllTaxRule->run();
             $taxRuleDataSets = explode(',', $this->taxRule);
             foreach ($taxRuleDataSets as $taxRuleDataSet) {
                 $taxRule = $this->fixtureFactory->createByCode(