Add test for `search_tap`.

Library/Homebrew/test/test_search_remote_tap.rb

+require "testing_env"
+require "cmd/search"
+
+class SearchRemoteTapTests < Homebrew::TestCase
+  def test_search_remote_tap
+    json_response = {
+      "tree" => [
+        {
+          "path" => "Formula/not-a-formula.rb",
+          "type" => "blob",
+        },
+      ],
+    }
+
+    GitHub.stubs(:open).yields(json_response)
+
+    assert_equal ["homebrew/not-a-tap/not-a-formula"], Homebrew.search_tap("homebrew", "not-a-tap", "not-a-formula")
+  end
+end