tests: set $COLUMNS when running CLI tests

As reported in jj-vcs#4394, at least `test_show_command::test_show_basic`
can fail when run with a narrow terminal. This patch sets
`COLUMNS=100` in the environment when running tests so the CLI uses
that value instead of using the width of the user's terminal.

cli/tests/common/mod.rs

@@ -89,6 +89,7 @@ impl TestEnvironment {
         cmd.current_dir(current_dir);
         cmd.args(args);
         cmd.env_clear();
+        cmd.env("COLUMNS", "100");
         for (key, value) in &self.env_vars {
             cmd.env(key, value);
         }