api/formula: Fix failing specs

- clear the formula API cache
- make the API cache directory
- fix stubbed return values (thanks Sorbet!)

Library/Homebrew/tap.rb

@@ -126,6 +126,7 @@ def clear_cache
  @formula_dir = nil
  @cask_dir = nil
  @command_dir = nil
+ @formula_names = nil
  @formula_files = nil
  @cask_files = nil
  @alias_dir = nil

Library/Homebrew/test/api/internal_tap_json/formula_spec.rb

@@ -31,7 +31,23 @@
 
  allow(Homebrew::API).to receive(:fetch_json_api_file)
  .with("internal/v3/homebrew-core.jws.json")
- .and_return(JSON.parse(internal_tap_json), false)
+ .and_return([JSON.parse(internal_tap_json), false])
+
+ # `Tap.reverse_tap_migrations_renames` looks for renames in every
+ # tap so `CoreCaskTap.tap_migrations` gets called and tries to
+ # fetch stuff from the API. This just avoids errors.
+ allow(Homebrew::API).to receive(:fetch_json_api_file)
+ .with("cask_tap_migrations.jws.json", anything)
+ .and_return([{}, false])
+
+ # To allow `formula_names.txt` to be written to the cache.
+ (HOMEBREW_CACHE/"api").mkdir
+
+ Homebrew::API::Formula.clear_cache
+ end
+
+ after do
+ Homebrew::API::Formula.clear_cache
  end
 
  it "loads tap aliases" do