Add tests for --source-code-external-url option

Author: GuillaumeGomez

src/test/rustdoc-ui/invalid_source_code_external_url.rs

@@ -0,0 +1,3 @@
+// compile-flags:--Z unstable-options --source-code-external-url hello
+
+pub struct Foo;

src/test/rustdoc-ui/invalid_source_code_external_url.stderr

@@ -0,0 +1,2 @@
+error: option `--source-code-external-url` argument must be an URL
+

src/test/rustdoc/source_code_external_url.rs

@@ -0,0 +1,8 @@
+// ignore-tidy-linelength
+// compile-flags: -Z unstable-options --source-code-external-url https://a.a
+
+#![crate_name = "foo"]
+
+// @has foo/struct.Foo.html
+// @has - '//h1[@class="fqn"]//a[@href="https://a.a/foo/source_code_external_url.rs.html#8"]' '[src]'
+pub struct Foo;

src/test/rustdoc/source_code_external_url2.rs

@@ -0,0 +1,8 @@
+// ignore-tidy-linelength
+// compile-flags: -Z unstable-options --source-code-external-url https://a.a/
+
+#![crate_name = "foo"]
+
+// @has foo/struct.Foo.html
+// @has - '//h1[@class="fqn"]//a[@href="https://a.a/foo/source_code_external_url2.rs.html#8"]' '[src]'
+pub struct Foo;