test: actually test tty getColorDepth()

TTY tests should almost never be placed in `/parallel/`. Skipping TTY
tests there due to missing tty fds just means they will never be run,
ever, on any system.

This moves the tty-get-color-depth test to `/pseudo-tty/` where the test
runner will actually make a pty fd.

Refs: nodejs/node#17615
PR-URL: nodejs/node#18800
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/pseudo-tty/test-tty-get-color-depth.js

@@ -0,0 +1,36 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert').strict;
+/* eslint-disable no-restricted-properties */
+const { WriteStream } = require('tty');
+
+const fd = common.getTTYfd();
+const writeStream = new WriteStream(fd);
+
+{
+  const depth = writeStream.getColorDepth();
+
+  assert.equal(typeof depth, 'number');
+  assert(depth >= 1 && depth <= 24);
+
+  if (depth === 1) {
+    // Terminal does not support colors, compare to a value that would.
+    assert.notEqual(writeStream.getColorDepth({ COLORTERM: '1' }), depth);
+  } else {
+    // Terminal supports colors, compare to a value that would not.
+    assert.notEqual(writeStream.getColorDepth({ TERM: 'dumb' }), depth);
+  }
+}
+
+// Deactivate colors
+{
+  const tmp = process.env.NODE_DISABLE_COLORS;
+  process.env.NODE_DISABLE_COLORS = 1;
+
+  const depth = writeStream.getColorDepth();
+
+  assert.equal(depth, 1);
+
+  process.env.NODE_DISABLE_COLORS = tmp;
+}