allow test_raise_subproc_error_with_show_traceback to be run locally (#5461)

previously, this succeeded in CI, but always failed locally with the
following error:
```xsh
_____________________________________________________________________ test_raise_subproc_error_with_show_traceback[True] _____________________________________________________________________

monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x76f6ba8882e0>, interactive = True

    @skip_if_on_windows
    @pytest.mark.parametrize("interactive", [True, False])
    def test_raise_subproc_error_with_show_traceback(monkeypatch, interactive):
        out, err, ret = run_xonsh(
            "$COLOR_RESULTS=False\n$RAISE_SUBPROC_ERROR=False\n$XONSH_SHOW_TRACEBACK=False\nls nofile",
            interactive=interactive,
            single_command=True,
        )
        assert ret != 0
>       assert re.match("ls.*No such file or directory\n", out)
E       assert None
E        +  where None = <function match at 0x76f6bcd0cd30>('ls.*No such file or directory\n', "\x1b]0;ls | jyn@pop-os: ~/src/xonsh | xonsh\x07ls: cannot access 'nofile': No such file or directory\n")
E        +    where <function match at 0x76f6bcd0cd30> = re.match

tests/test_integrations.py:1190: AssertionError
```

the problem was that the following string was prepended before the error
message:
```
\x1b]0;ls | jyn@pop-os: ~/src/xonsh | xonsh\x07l
```

that in turn came from
[`BaseShell.settitle`](66c0490d37/xonsh/base_shell.py (L570)),
which always prepends at least the escape codes even when TITLE is set
to an empty string.

avoid this issue by mimicking a dumb terminal in tests, in which case
`settitle` doesn't add any title at all.

see
https://xonsh.zulipchat.com/#narrow/stream/435069-xonsh-dev/topic/Running.20tests.20locally
for more context.
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comment**

tests/test_integrations.py

@@ -63,6 +63,8 @@ def run_xonsh(
     env["RAISE_SUBPROC_ERROR"] = "0"
     env["FOREIGN_ALIASES_SUPPRESS_SKIP_MESSAGE"] = "1"
     env["PROMPT"] = ""
+    # disable ansi escape codes
+    env["TERM"] = "linux"
     xonsh = shutil.which("xonsh", path=PATH)
     args = [xonsh, "--no-rc"]
     if interactive: