Revise prefix a bit, adding both --test (conditionally) and [RUSTC-SHIM] unconditionally.

1. I added `--test` based on review feedback from simulacrum: I decided I would
rather include such extra context than get confused later on by its absence.
(However, I chose to encode it differently than how `[RUSTC-TIMING]` does... I
don't have much basis for doing so, other than `--test` to me more directly
reflects what it came from.)

2. I also decided to include `[RUSTC-SHIM]` at start of all of these lines
driven by the verbosity level, to make to clear where these lines of text
originate from. (Basically, I skimmed over the output and realized that a casual
observer might not be able to tell where this huge set of new lines were coming
from.)

Author: pnkfelix

src/bootstrap/bin/rustc.rs

@@ -138,12 +138,14 @@ fn main() {
         cmd.arg("-Z").arg("force-unstable-if-unmarked");
     }
 
+    let is_test = args.iter().any(|a| a == "--test");
     if verbose > 1 {
         let rust_env_vars =
             env::vars().filter(|(k, _)| k.starts_with("RUST") || k.starts_with("CARGO"));
+        let prefix = if is_test { "[RUSTC-SHIM] rustc --test" } else { "[RUSTC-SHIM] rustc" };
         let prefix = match crate_name {
-            Some(crate_name) => format!("rustc {}", crate_name),
-            None => "rustc".to_string(),
+            Some(crate_name) => format!("{} {}", prefix, crate_name),
+            None => prefix.to_string(),
         };
         for (i, (k, v)) in rust_env_vars.enumerate() {
             eprintln!("{} env[{}]: {:?}={:?}", prefix, i, k, v);
@@ -171,7 +173,6 @@ fn main() {
     {
         if let Some(crate_name) = crate_name {
             let dur = start.elapsed();
-            let is_test = args.iter().any(|a| a == "--test");
             // If the user requested resource usage data, then
             // include that in addition to the timing output.
             let rusage_data =