Start implementing a test suite for the CLI

We have very few tests today so this starts to add the basics of a test
suite which compiles Cargo projects on-the-fly which will hopefully help
us bolster the amount of assertions we can make about the output.

crates/cli/src/bin/wasm-bindgen.rs

@@ -79,7 +79,7 @@ fn main() {
     };
     eprintln!("error: {}", err);
     for cause in err.iter_causes() {
-        eprintln!("\tcaused by: {}", cause);
+        eprintln!("    caused by: {}", cause);
     }
     process::exit(1);
 }