Update PreferencesController#_validateERC20AssetParams tests (#9204)

This change fixes the `_validateERC20AssetParams` tests, ensuring that the given
options are all valid except those that are being tested. Previously the `symbol`
property was invalid _in addition to_ the `decimals` property.

test/unit/app/controllers/preferences-controller-test.js

@@ -485,8 +485,8 @@ describe('preferences controller', function () {
       assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', decimals: 0 }), 'missing symbol should fail')
       assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', symbol: 'ABC' }), 'missing decimals should fail')
       assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', symbol: 'ABCDEFGHI', decimals: 0 }), 'invalid symbol should fail')
-      assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', symbol: 'ABCDEFGHI', decimals: -1 }), 'decimals < 0 should fail')
-      assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', symbol: 'ABCDEFGHI', decimals: 38 }), 'decimals > 36 should fail')
+      assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', symbol: 'ABC', decimals: -1 }), 'decimals < 0 should fail')
+      assert.throws(() => validate({ rawAddress: '0xd26114cd6EE289AccF82350c8d8487fedB8A0C07', symbol: 'ABC', decimals: 38 }), 'decimals > 36 should fail')
       assert.throws(() => validate({ rawAddress: '0x123', symbol: 'ABC', decimals: 0 }), 'invalid address should fail')
     })
   })