test: don't fail if GitHub API is unavailable

Tests shouldn't fail in case of an unavailable network or a deliberately
disabled access to the GitHub API.

Closes Homebrew/homebrew#47670.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/test/test_tap.rb

@@ -5,6 +5,7 @@ class TapTest < Homebrew::TestCase
     @path = Tap::TAP_DIRECTORY/"homebrew/homebrew-foo"
     @path.mkpath
     @tap = Tap.new("Homebrew", "foo")
+    @tap.stubs(:private?).returns(true) if ENV["HOMEBREW_NO_GITHUB_API"]
   end
 
   def setup_tap_files