Add test for no src links on dummy spans

Author: GuillaumeGomez

src/test/rustdoc/src-links-auto-impls.rs

@@ -0,0 +1,12 @@
+#![crate_name = "foo"]
+
+// @has foo/struct.Unsized.html
+// @has - '//h3[@id="impl-Sized"]/code' 'impl !Sized for Unsized'
+// @!has - '//h3[@id="impl-Sized"]/a[@class="srclink"]' '[src]'
+// @has - '//h3[@id="impl-Sync"]/code' 'impl Sync for Unsized'
+// @!has - '//h3[@id="impl-Sync"]/a[@class="srclink"]' '[src]'
+// @has - '//h3[@id="impl-Any"]/code' 'impl<T> Any for T'
+// @has - '//h3[@id="impl-Any"]/a[@class="srclink"]' '[src]'
+pub struct Unsized {
+    data: [u8],
+}