tests: drop an unnecessary redirect to /dev/null

This eliminates the need to conditionalize the color output, because NUL on
Windows is a tty.  Credit to Yuya for recognizing this.

tests/test-extension.t

@@ -1254,11 +1254,9 b' Broken disabled extension and command:'
   > def g():
   >     pass
   > EOF
-  $ hg --config extensions.path=./path.py help foo > /dev/null
-  abort: no such help topic: foo (no-windows !)
-  (try 'hg help --keyword foo') (no-windows !)
-  \x1b[0;31mabort: no such help topic: foo\x1b[0m (esc) (windows !)
-  \x1b[0;31m(try 'hg help --keyword foo')\x1b[0m (esc) (windows !)
+  $ hg --config extensions.path=./path.py help foo
+  abort: no such help topic: foo
+  (try 'hg help --keyword foo')
   [255]
 
   $ cat > throw.py <<EOF

tests/test-help.t

@@ -1480,11 +1480,9 b' Test -e / -c / -k combinations'
   Commands:
   $ hg help -c commit > /dev/null
   $ hg help -e -c commit > /dev/null
-  $ hg help -e commit > /dev/null
-  abort: no such help topic: commit (no-windows !)
-  (try 'hg help --keyword commit') (no-windows !)
-  \x1b[0;31mabort: no such help topic: commit\x1b[0m (esc) (windows !)
-  \x1b[0;31m(try 'hg help --keyword commit')\x1b[0m (esc) (windows !)
+  $ hg help -e commit
+  abort: no such help topic: commit
+  (try 'hg help --keyword commit')
   [255]
 
 Test keyword search help