rust: add tests for safe keyword in unsafe extern block

Author: fw-immunant

test/markup/rust/extern.expect.txt

@@ -0,0 +1,4 @@
+<span class="hljs-keyword">unsafe</span> <span class="hljs-keyword">extern</span> <span class="hljs-string">&quot;C&quot;</span> {
+<span class="hljs-keyword">unsafe</span> <span class="hljs-keyword">fn</span> <span class="hljs-title function_">foo</span>();
+safe <span class="hljs-keyword">fn</span> <span class="hljs-title function_">foo</span>();
+}

test/markup/rust/extern.txt

@@ -0,0 +1,4 @@
+unsafe extern "C" {
+unsafe fn foo();
+safe fn foo();
+}