Fix commented out part of test_body_encode in test_email #149916

It was commented out in bef9d21. The test was broken because:

• Charset.body_encode() now only takes one argument (see 9604e66).
• The argument of Charset.body_encode() must be Unicode string for encoding taking place. The test used a 8-bit string in Python 2.

So the test can only be restored partially, but it was the only test with different input and output codecs, so it is better to enable it.

Linked PRs

• gh-149916: Restore the commented out part of test_body_encode in test_email #149917
• [3.15] gh-149916: Restore the commented out part of test_body_encode in test_email (GH-149917) #149937
• [3.14] gh-149916: Restore the commented out part of test_body_encode in test_email (GH-149917) #149938
• [3.13] gh-149916: Restore the commented out part of test_body_encode in test_email (GH-149917) #149939