Add partial body to failing uses of bodyContains for debugging

[MaxGabriel]

When statusIs fails and the content is UTF-8 (i.e. like, not a giant JPEG or something), we print a partial body to aid in debugging. For small bodies this is sometimes sufficient to print the whole thing, and especially is helpful to indicate a general direction if the body is way off (e.g. the body for a 500 vs a 200).

This PR applies that same helpful error message to failing bodyContains and bodyNotContains calls.

A potential variant would be to print the full body, which bodyEquals does. That could be more helpful, but there is also some risk that bodyContains and bodyNotContains are being used because it is an enormous response body, and printing the full thing would dump a lot of content.

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[MaxGabriel]

Tested this locally just by modifying a test in yesod-test's own suite. Example error messages:

Expected body to contain `colorCheckBoxes = [Red,Gray]`. For debugging, the body was: CheckboxesForm {colorCheckBoxes = [Red]}

Expected body to contain `colorCheckBoxes = [Blue]`. For debugging, the body was: CheckboxesForm {colorCheckBoxes = [Red,Gray]}

[parsonsmatt]

Hell yeah

[MaxGabriel]

Released https://hackage.haskell.org/package/yesod-test-1.7.0.1/changelog