jest-snapshot: Remove only the added newlines in multiline snapshots

[pedrottimark]

Summary

Because a custom serializer might return a string with newlines or spaces at beginning or end, trim is not the correct method (pardon pun :) to remove the two newlines that were added at beginning and end of multiline content.

Example of the problem: #8850 (comment) and #8850 (comment)

Test plan

Added 7 tests

See also pictures in the following comment for local end-to-end test

[pedrottimark]

Example pictures baseline at left and improved at right

without serializer is enclosed in double quote marks, so baseline and improved both correct:

const expected = ['first line', 'second line', ''].join('\n');
const received = ['first line', 'second line'].join('\n');

with serializer which joins array items with newline has no punctuation to stop the over-trim:

const expected = ['first line', 'second line', ''];
const received = ['first line', 'second line'];

[SimenB]

What an elegant solution! Thanks!! 😀

[github-actions]

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