try to bring some consistency to terminal-width tests

tests/test_pytest_icdiff.py

@@ -1,4 +1,6 @@
 import icdiff
+from unittest import mock
+import pytest
 import re
 from pprintpp import pformat
 
@@ -7,6 +9,13 @@
 GREEN_ON = '\x1b[1;32m'
 ANSI_ESCAPE_RE = re.compile(r'(\x9B|\x1B\[)[0-?]*[ -/]*[@-~]')
 
+@pytest.fixture
+def with_80_col_terminal():
+    # haven't found a better way to simulate terminal sizes
+    # at a lower level yet sadly.
+    with mock.patch("pytest_icdiff.COLS", 99):
+        yield
+
 
 def test_short_dict(testdir):
     one = {
@@ -193,6 +202,7 @@ def test_a():
     assert drilldown_expression in output
 
 
+@pytest.mark.usefixtures("with_80_col_terminal")
 def test_long_lines_in_comparators_are_wrapped_sensibly_multiline(testdir):
     left = {1: "hello " * 20, 2: 'two'}
     right = {1: "hella " * 20, 2: 'two'}
@@ -207,6 +217,7 @@ def test_one():
     assert comparison_line.count('hell') < 13
 
 
+@pytest.mark.usefixtures("with_80_col_terminal")
 def test_long_lines_in_comparators_are_wrapped_sensibly_singleline(testdir):
     left = "hello " * 10
     right = "hella " * 10
@@ -277,6 +288,7 @@ def test_one():
     assert f"123456   123456{GREEN_ON}7" in output
 
 
+@pytest.mark.usefixtures("with_80_col_terminal")
 def test_really_long_diffs_use_context_mode(testdir):
     testdir.makepyfile(
         f"""