Add TextEmitter::with_color and disable colors in unreadable_files test (#19562)

Summary
--

I looked at other uses of `TextEmitter`, and I think this should be the
only one affected by this. The other integration tests must work
properly since they're run with `assert_cmd_snapshot!`, which I assume
triggers the `SHOULD_COLORIZE` case, and the `cfg!(test)` check will
work for uses in `ruff_linter`.


4a4dc38b5b/crates/ruff_linter/src/message/text.rs (L36-L44)

Alternatively, we could probably move this to a CLI test instead.

Test Plan
--

`cargo test -p ruff`, which was failing on `main` with color codes in
the output before this

crates/ruff/src/commands/check.rs

@@ -279,6 +279,7 @@ mod test {
 
         TextEmitter::default()
             .with_show_fix_status(true)
+            .with_color(false)
             .emit(
                 &mut output,
                 &diagnostics.inner,

crates/ruff_linter/src/message/text.rs

@@ -76,6 +76,12 @@ impl TextEmitter {
         self.config = self.config.preview(preview);
         self
     }
+
+    #[must_use]
+    pub fn with_color(mut self, color: bool) -> Self {
+        self.config = self.config.color(color);
+        self
+    }
 }
 
 impl Emitter for TextEmitter {