Fix ANSI escape codes for colored output not handled correctly with pytest.fail(..., pytrace=False)

[leonarduschen]

Closes #12849

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[leonarduschen]

Added this so that calling get_write_msg raises AssertionError instead of ValueError: Not enough values to unpack

I can revert it back if needed

[leonarduschen]

Using TerminalWriter.write instead of TerminalWriter.line because when using TWMock . we can't distinguish lines written with TWMock._write_source and lines written with line directly, whereas with TWMock.write we can, with the flag.

We could add a flag for TWMock._write_source as well, like so:

class TWMock:
    WRITE = object()
    WRITE_SOURCE = object()

    def _write_source(self, lines, indents=()):
        if not indents:
            indents = [""] * len(lines)
        for indent, line in zip(indents, lines):
            newline = indent + line
            self.line((TWMock.WRITE_SOURCE, newline))

But then we'll have to change around half the test cases in test_excinfo.py