Add windows to test matrix

[Wuodan]

Changes

This adds windows-latest to the test matrix for npm tests.
With the second commit, the npm tests are executed by npx to avoid ${CI+-t} path expansion failure on Windows.

Reason

bats-assert and bats-support should run tests on Windows themselves.
Right now such errors only surface in the tests of bats-action.

The test for bats-core/bats-action on Windows currently fail because bats-action by default uses an older version of bats-assert (v2.1.0, hardcoded in input defaults in action.yml:41) which does not have the fix from commit https://github.com/bats-core/bats-action/blob/562ebf679115bde788f656cd532408c2d125297d/action.yaml#L41 to bats-action.
The update in bats-action is trivial, but this should be detected by tests in bats-assert.

[Wuodan]

Update:
Earlier in this commit I suggested running tests with npx to make them work on Windows.
But actually the issues were caused by bats-support and bats-assert themselves and can easily be fixed there.

Related PRs:

• PR 23 in bats-core/bats-support
• PR 99 in bats-core/bats-assert