Fix test_edit to work with BSD sed

[mtelka]

Fixes #3128.

[Rowlando13]

Please add a comment.

[mtelka]

Please add a comment.

@Rowlando13 could you please elaborate what kind of comment you mean? Thank you.

[Rowlando13]

Explain in short why the test was altered. Specifically note the os and reason. If you don't we may unintentionally break the test again or add a similar breaking test.

[mtelka]

Explain in short why the test was altered. Specifically note the os and reason. If you don't we may unintentionally break the test again or add a similar breaking test.

Here in the PR, or in the sources?

[Rowlando13]

If by sources you mean in tests/test_termui.py, then yes.

[mtelka]

If by sources you mean in tests/test_termui.py, then yes.

Okay, will do.

In the meantime, POSIX says this about sed:

In default operation, sed cyclically shall append a line of input, less its 
terminating <newline> character, into the pattern space. Reading from input
shall be skipped if a <newline> was in the pattern space prior to a D command 
ending the previous cycle. The sed utility shall then apply in sequence all 
commands whose addresses select that pattern space, until a command starts the
next cycle or quits. If no commands explicitly started a new cycle, then at the
end of the script the pattern space shall be copied to standard output (except
when -n is specified) and the pattern space shall be deleted. Whenever the
pattern space is written to standard output or a named file, sed shall
immediately follow it with a <newline>.

So it looks like GNU sed does not follow this accurately.

[mtelka]

Please add a comment.

Done. @Rowlando13 please let me know if you'd prefer different wording there. Thank you.