dev-cmd/tests: skip tests that require core if it's not tapped

There were a few tests which require core to be tapped and fail
if it isn't. This is annoying if someone is trying to contribute
to the project and they're using the JSON API instead of having
the core repo tapped locally.

I'm just skipping these because it's the simplest thing to do.
The tests that failed are mostly rubocop tests so it's fine
if they only run on CI.

Library/Homebrew/test/cmd/--prefix_spec.rb

@@ -13,7 +13,7 @@ RSpec.describe Homebrew::Cmd::Prefix do
       .and be_a_success
   end
 
-  it "prints the prefix for a Formula", :integration_test do
+  it "prints the prefix for a Formula", :integration_test, :needs_homebrew_core do
     expect { brew_sh "--prefix", "wget" }
       .to output("#{ENV.fetch("HOMEBREW_PREFIX")}/opt/wget\n").to_stdout
       .and not_to_output.to_stderr

Library/Homebrew/test/spec_helper.rb

@@ -160,6 +160,11 @@ RSpec.configure do |config|
     skip "Requires network connection." unless ENV["HOMEBREW_TEST_ONLINE"]
   end
 
+  config.before(:each, :needs_homebrew_core) do
+    core_tap_path = "#{ENV.fetch("HOMEBREW_LIBRARY")}/Taps/homebrew/homebrew-core"
+    skip "Requires homebrew/core to be tapped." unless Dir.exist?(core_tap_path)
+  end
+
   config.before do |example|
     next if example.metadata.key?(:needs_network)
     next if example.metadata.key?(:needs_utils_curl)

Library/Homebrew/test/support/helper/spec/shared_examples/formulae_exist.rb

@@ -2,7 +2,7 @@
 
 RSpec.shared_examples "formulae exist" do |array|
   array.each do |f|
-    it "#{f} formula exists" do
+    it "#{f} formula exists", :needs_homebrew_core do
       core_tap = Pathname("#{HOMEBREW_LIBRARY_PATH}/../Taps/homebrew/homebrew-core")
       formula_paths = core_tap.glob("Formula/**/#{f}.rb")
       alias_path = core_tap/"Aliases/#{f}"