Fixed the "==" constraint to a.i == b.i in PrepareSpec."with outer join"

[b-gyula]

Problem

Test case PrepareSpec."with outer join" was creating a meaningless join (a, b) <- qr1 join qr2 on ((a, b) => a.i == a.i)

Solution

Fixed the "==" constraint to a.i == b.i

Checklist

• Unit test all changes
• Update README.md if applicable
• Add [WIP] to the pull request title if it's work in progress
• Squash commits that aren't meaningful changes
• Run sbt scalariformFormat test:scalariformFormat to make sure that the source files are formatted

@getquill/maintainers

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