Add spec tests for GitLab raw content include processor

Even though, it's untested since I barely use it anymore.

spec/gitlab_raw_content_include_processor_spec.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+describe GitLabRawIncludeProcessor, if: ENV['GITLAB_API_PERSONAL_ACCESS_TOKEN'] do
+  it 'should include the GitLab CI configuration from freedesktop-sdk/freedesktop-sdk from the default instance' do
+    commit = 'bcb3e0de957519e87a4c7b8c0e40af9876e531e7'
+    input = <<~INPUT
+      [literal]
+      ....
+      include::gitlab:freedesktop-sdk/freedesktop-sdk[rev=#{commit}, path=.gitlab-ci.yml]
+      ....
+    INPUT
+
+    expected = File.read(fixtures_file("freedesktop-sdk-#{commit}-.gitlab-ci.yml"))
+
+    actual = (Asciidoctor.convert input).tr_s '\n', '\n'
+    (expect actual).to include expected.chomp
+  end
+
+  it 'should include the Meson options of Warp from the GNOME GitLab instance' do
+    commit = 'v0.5.2'
+    input = <<~INPUT
+      [literal]
+      ....
+      include::World/warp[domain=gitlab.gnome.org, rev=#{commit}, path=meson_options.txt]
+      ....
+    INPUT
+
+    expected = File.read(fixtures_file("warp-#{commit}-meson_options.txt"))
+
+    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 a repo' do
+    input = 'include::gitlab:freedesktop-sdk/freedesktop-sdk[]'
+
+    expect { Asciidoctor.convert input }.to raise_error StandardError
+  end
+
+  it 'should fail to point to non-existent repo' do
+    input = 'include::gitlab:nonexistentrepoyay/OKOKOKOKOKOKOK[path=README.adoc, rev=v2.0.0]'
+
+    expect { Asciidoctor.convert input }.to raise_error StandardError
+  end
+
+  it 'should fail to point to non-existent repo and a non-existent instance' do
+    input = 'include::gitlab:nonexistentrepoyay/OKOKOKOKOKOKOK[domain=example.com, path=README.adoc, rev=v2.0.0]'
+
+    expect { Asciidoctor.convert input }.to raise_error StandardError
+  end
+end