Fix prop types for SendGasRow component tests

[whymarrh]

This PR adds required props to the SendGasRow component tests to reduce noise in the output.

Before:

$ yarn test:unit
yarn run v1.19.2
$ cross-env METAMASK_ENV=test mocha --exit --require test/setup.js --recursive "test/unit/**/*.js" "ui/app/**/*.test.js"


  SendGasRow Component
    render
Warning: Failed prop type: Invalid prop `gasFeeError` of type `string` supplied to `SendGasRow`, expected `boolean`.
    in SendGasRow
Warning: Failed prop type: Invalid prop `showError` of type `string` supplied to `SendRowWrapper`, expected `boolean`.
    in SendRowWrapper
      ✓ should render a SendRowWrapper component
      ✓ should pass the correct props to SendRowWrapper
      ✓ should render a GasFeeDisplay as a child of the SendRowWrapper
      ✓ should render the GasFeeDisplay
      ✓ should render the GasPriceButtonGroup if gasButtonGroupShown is true
      ✓ should render an advanced options button if gasButtonGroupShown is true


  6 passing (344ms)

✨  Done in 20.66s.

After:

$ yarn test:unit
yarn run v1.19.2
$ cross-env METAMASK_ENV=test mocha --exit --require test/setup.js --recursive "test/unit/**/*.js" "ui/app/**/*.test.js"


  SendGasRow Component
    render
      ✓ should render a SendRowWrapper component
      ✓ should pass the correct props to SendRowWrapper
      ✓ should render a GasFeeDisplay as a child of the SendRowWrapper
      ✓ should render the GasFeeDisplay
      ✓ should render the GasPriceButtonGroup if gasButtonGroupShown is true
      ✓ should render an advanced options button if gasButtonGroupShown is true


  6 passing (342ms)

✨  Done in 19.00s.

[Gudahtt]

LGTM!