Add a _spendAllowance function to ERC20 & ERC777 (#3170)

(cherry picked from commit c5a6cae898)

test/token/ERC20/extensions/ERC20Burnable.behavior.js

@@ -98,7 +98,7 @@ function shouldBehaveLikeERC20Burnable (owner, initialBalance, [burner]) {
       it('reverts', async function () {
         await this.token.approve(burner, allowance, { from: owner });
         await expectRevert(this.token.burnFrom(owner, allowance.addn(1), { from: burner }),
-          'ERC20: burn amount exceeds allowance',
+          'ERC20: insufficient allowance',
         );
       });
     });

test/token/ERC20/extensions/ERC20Wrapper.test.js

@@ -59,7 +59,7 @@ contract('ERC20', function (accounts) {
     it('missing approval', async function () {
       await expectRevert(
         this.token.depositFor(initialHolder, initialSupply, { from: initialHolder }),
-        'ERC20: transfer amount exceeds allowance',
+        'ERC20: insufficient allowance',
       );
     });