test: disable colors in test-assert.js

When test/parallel/test-assert.js is run with a TTY as stdout,
color codes in assertion messages cause the test to fail. This
commit disables colors when stdout is a TTY.

Fixes: #18967
PR-URL: #20695
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-assert.js

@@ -34,6 +34,12 @@ const { writeFileSync, unlinkSync } = require('fs');
 const { inspect } = require('util');
 const a = assert;
 
+// Disable colored output to prevent color codes from breaking assertion
+// message comparisons. This should only be an issue when process.stdout
+// is a TTY.
+if (process.stdout.isTTY)
+  process.env.NODE_DISABLE_COLORS = '1';
+
 const start = 'Input A expected to strictly deep-equal input B:';
 const actExp = '+ expected - actual';