test2json: tolerate \r\n as a newline separator in test output

[nd]

This allows to debug tests and parse their status with test2json on
mac.

On mac debugserver sets up a pseudo terminal for test executable under
debug. The terminal has the ONLCR option enabled which turns '\n' in
go test output into '\r\n'. Test2json doesn't expect '\r\n' as a new
line and reports no test status.

This commit changes test2json so that it handles 'PASS\r\n' and
'FAIL\r\n' in the same way as 'PASS\n' and 'FAIL\n'.

Fixes #34286

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