missing_formula: TexLive is only blacklisted on macOS

- In Homebrew/linuxbrew-core, [we have a Linux-only formula for
  `texlive`](https://github.com/Homebrew/linuxbrew-core/tree/master/Formula/texlive.rb).
- When running `brew audit --strict texlive` on Linux, we got the
  following messaging:

  ```
  $ brew audit --strict texlive
  texlive:
    * 'texlive' is blacklisted from homebrew/core.
    Error: 1 problem in 1 formula detected
  ```

- Looking at where this comes from leads to the missing formula
  messaging to install `mactex` via Homebrew Cask. The 'blacklisted in
  homebrew/core' messaging only applies to macOS where Casks are an option
  for users, so let's not surface the audit for `texlive` on Linux.

Library/Homebrew/extend/os/mac/missing_formula.rb

@@ -14,6 +14,19 @@ module Homebrew
           <<~EOS
             Xcode can be installed from the App Store.
           EOS
+        when "tex", "tex-live", "texlive", "mactex", "latex"
+          <<~EOS
+            There are three versions of MacTeX.
+
+            Full installation:
+              brew cask install mactex
+
+            Full installation without bundled applications:
+              brew cask install mactex-no-gui
+
+            Minimal installation:
+              brew cask install basictex
+          EOS
         else
           generic_blacklisted_reason(name)
         end

Library/Homebrew/missing_formula.rb

@@ -16,18 +16,6 @@ module Homebrew
           macOS provides gem as part of Ruby. To install a newer version:
             brew install ruby
         EOS
-        when "tex", "tex-live", "texlive", "mactex", "latex" then <<~EOS
-          There are three versions of MacTeX.
-
-          Full installation:
-            brew cask install mactex
-
-          Full installation without bundled applications:
-            brew cask install mactex-no-gui
-
-          Minimal installation:
-            brew cask install basictex
-        EOS
         when "pip" then <<~EOS
           pip is part of the python formula:
             brew install python

Library/Homebrew/test/missing_formula_spec.rb

@@ -17,7 +17,7 @@ describe Homebrew::MissingFormula do
     end
 
     it { is_expected.to blacklist("gem") }
-    it { is_expected.to blacklist("latex") }
+    it("blacklists LaTeX", :needs_macos) { is_expected.to blacklist("latex") }
     it { is_expected.to blacklist("pip") }
     it { is_expected.to blacklist("pil") }
     it { is_expected.to blacklist("macruby") }