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 @@ class Tap
     @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 @@ RSpec.describe "Internal Tap JSON -- Formula" do
 
       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