Improve testing framework for http registries

Improve integration of the http server introduced by the http-registry feature.
Now the same HTTP server is used for serving downloads, the index, and
the API.

This makes it easier to write tests that deal with authentication and
http registries.

crates/cargo-test-support/src/lib.rs

@@ -569,6 +569,12 @@ impl Execs {
         self
     }
 
+    /// Writes the given lines to stdin.
+    pub fn with_stdin<S: ToString>(&mut self, expected: S) -> &mut Self {
+        self.expect_stdin = Some(expected.to_string());
+        self
+    }
+
     /// Verifies the exit code from the process.
     ///
     /// This is not necessary if the expected exit code is `0`.
@@ -820,7 +826,10 @@ impl Execs {
     #[track_caller]
     pub fn run(&mut self) {
         self.ran = true;
-        let p = (&self.process_builder).clone().unwrap();
+        let mut p = (&self.process_builder).clone().unwrap();
+        if let Some(stdin) = self.expect_stdin.take() {
+            p.stdin(stdin);
+        }
         if let Err(e) = self.match_process(&p) {
             panic_error(&format!("test failed running {}", p), e);
         }