Added markup test for uppercase string sigil with heredoc

Relates to p.2 from highlightjs#730

Unfortunately, we do not distinguish s and S sigils with different interpolation rules

test/markup/elixir/uppercase-string-sigil.expect.txt

@@ -0,0 +1,17 @@
+<span class="hljs-class"><span class="hljs-keyword">defmodule</span> <span class="hljs-title">Long</span></span>.Module.Name <span class="hljs-keyword">do</span>
+  <span class="hljs-variable">@doc</span> ~S<span class="hljs-string">''</span><span class="hljs-string">'
+  No <span class="hljs-subst">#{interpolation}</span> of any kind.
+  \000 \x{ff}
+
+  \n <span class="hljs-subst">#{\x{ff}</span>}
+  '</span><span class="hljs-string">''</span>
+  <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">func</span></span>(a, b \\ []), <span class="hljs-symbol">do:</span> <span class="hljs-symbol">:ok</span>
+
+  <span class="hljs-variable">@doc</span> ~S<span class="hljs-string">""</span><span class="hljs-string">"
+  No <span class="hljs-subst">#{interpolation}</span> of any kind.
+  \000 \x{ff}
+
+  \n <span class="hljs-subst">#{\x{ff}</span>}
+  "</span><span class="hljs-string">""</span>
+  <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">func</span></span>(a, b \\ []), <span class="hljs-symbol">do:</span> <span class="hljs-symbol">:ok</span>
+<span class="hljs-keyword">end</span>

test/markup/elixir/uppercase-string-sigil.txt

@@ -0,0 +1,17 @@
+defmodule Long.Module.Name do
+  @doc ~S'''
+  No #{interpolation} of any kind.
+  \000 \x{ff}
+
+  \n #{\x{ff}}
+  '''
+  def func(a, b \\ []), do: :ok
+
+  @doc ~S"""
+  No #{interpolation} of any kind.
+  \000 \x{ff}
+
+  \n #{\x{ff}}
+  """
+  def func(a, b \\ []), do: :ok
+end