Add spec tests for GitHub raw content include processor

spec/fixtures/asciidoctor-v2.0.0-Rakefile

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+# frozen_string_literal: true
+Dir.glob('tasks/*.rake').each {|file| load file }
+
+task default: %w(test:all)

spec/fixtures/nixpkgs-a46b89df91f56c8aca7de32d270422353d855483-COPYING

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+Copyright (c) 2003-2019 Eelco Dolstra and the Nixpkgs/NixOS contributors
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

spec/github_raw_content_include_processor_spec.rb

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+# frozen_string_literal: true
+
+describe GitHubRawIncludeProcessor do
+  it 'should include the raw content of a GitHub file successfully' do
+    input = <<~INPUT
+      [literal]
+      ....
+      include::github:asciidoctor/asciidoctor[path=Rakefile, rev=v2.0.0]
+      ....
+    INPUT
+
+    expected = File.read(fixtures_file('asciidoctor-v2.0.0-Rakefile'))
+
+    actual = Asciidoctor.convert input.tr_s '\n', '\n'
+    (expect actual).to include expected.chomp
+  end
+
+  it 'should get the raw content of COPYING from nixpkgs at a specific commit' do
+    commit = 'a46b89df91f56c8aca7de32d270422353d855483'
+    input = <<~INPUT
+      [literal]
+      ....
+      include::github:NixOS/nixpkgs[rev=#{commit}, path=COPYING]
+      ....
+    INPUT
+
+    expected = File.read(fixtures_file("nixpkgs-#{commit}-COPYING"))
+
+    actual = (Asciidoctor.convert input).tr_s '\n', '\n'
+    (expect actual).to include expected.chomp
+  end
+
+  it 'should not process since it points to the root directory of the repo' do
+    input = 'include::github:asciidoctor/asciidoctor[]'
+
+    expect { Asciidoctor.convert input }.to raise_error StandardError
+  end
+
+  it 'should fail to point to non-existent repo' do
+    input = 'include::github:nonexistentrepoyay/OKOKOKOKOKOKOK[path=README.adoc, rev=v2.0.0]'
+
+    expect { Asciidoctor.convert input }.to raise_error StandardError
+  end
+end