Rename the shadowing module from "ctor" to "core".

Given the description of the commit that added macro_hygiene.rs,
I believe that the name of the "ctor" module in this test should
be "core", verifying that macros work properly when a crate under
test has declared its own core module.

Author: carlwhamilton

integration_tests/src/macro_hygiene.rs

@@ -16,7 +16,7 @@ fn main() {}
 
 // Shadow important modules that the macros use.
 mod std {}
-mod ctor {}
+mod core {}
 
 #[cfg(test)]
 mod tests {