[chore]: upward-spec doc fixes

[sirugh]

You can ignore the description -- I was going to make some changes to the dependencies for upward-spec but decided against that. So this is just some docs changes to fix some issues I ran in when running the steps in the upward-spec readme.

Description

This PR is really in regards to the security issues reported on the upward-php repo.

That said, there is little we can actually do aside from not using tap-diff as the repo hasn't been updated in 4 years. Since we're indirectly using diff through tap-diff which is simply a test reporter (used when you run the upward-spec bin) I think we can resolve this security report as a won't fix. However, if people feel strongly enough we could remove the tap-diff dependency and output entirely. There are a lot of other options for tap output prettifiers but most are really outdated.

So the only change I made here was a fix to the npx command in the readme that wasn't working and to make the test script executable by default so that you can actually run the command.

Related Issue

Doesn't quite close PWA-126. For that we will need to do the steps listed in that ticket.

Verification steps

1. Run the script that I modified and see it run. Tests still fail but 🤷‍♂

[PWAStudioBot]

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📖	Associated JIRA tickets: PWA-126.

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