Running tests in the Svelte Typescript project does not fail due to compiler errors.

[CrowdHailer]

I've started a new project with the Svelte and typescript template.
I was just testing what help I would expect typescript to give me.

I have added this test

describe('<App>', () => {
  it('should fail type checks', () => {
    let x: number = 5;
    x = undefined;
    expect("").to.equal("")
  });
});

Note I have also turned on the strictNullChecks option in tsconfig.json.

When running npm test all the tests pass. However running npm run build fails because of the error of assigning undefined to the variable x.

I'm curious how the typescript tests are being compiled without the error to then be run.
Should a type error fail the test process?

I'm also curious why the test files are being build when running npm run build it seems like they should not be needed in the build process.
Should test files be transpiled as part of the build process?