Adds --bless support to test/run-make-fulldeps

The ability to "bless" output for some of these tests is critical to
making it practical to adapt tests to unrelated changes.

This is needed for new coverage tests, as shown in PR rust-lang#75828 (or its
derivative).

src/tools/compiletest/src/runtest.rs

@@ -2782,6 +2782,18 @@ impl<'test> TestCx<'test> {
             cmd.env("RUSTFLAGS", "-Ctarget-feature=-crt-static").env("IS_MUSL_HOST", "1");
         }
 
+        if self.config.bless {
+            cmd.env("RUSTC_BLESS_TEST", "--bless");
+            // Assume this option is active if the environment variable is "defined", with _any_ value.
+            // As an example, a `Makefile` can use this option by:
+            //
+            //   ifdef RUSTC_BLESS_TEST
+            //       cp "$(TMPDIR)"/actual_something.ext expected_something.ext
+            //   else
+            //       $(DIFF) expected_something.ext "$(TMPDIR)"/actual_something.ext
+            //   endif
+        }
+
         if self.config.target.contains("msvc") && self.config.cc != "" {
             // We need to pass a path to `lib.exe`, so assume that `cc` is `cl.exe`
             // and that `lib.exe` lives next to it.