compiletest: add specific //@ ignore-auxiliary for test support files

This is for files that *participate* in actual tests but should not be
built by `compiletest` (i.e. these files are involved through `mod xxx;`
or `include!()` or `#[path = "xxx"]`, etc.).

A specialized directive like `//@ ignore-auxiliary` makes it way easier
to audit disabled tests via `//@ ignore-test`.

src/tools/compiletest/src/directive-list.rs

@@ -44,6 +44,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "ignore-arm-unknown-linux-gnueabihf",
     "ignore-arm-unknown-linux-musleabi",
     "ignore-arm-unknown-linux-musleabihf",
+    "ignore-auxiliary",
     "ignore-avr",
     "ignore-beta",
     "ignore-cdb",

src/tools/compiletest/src/header/cfg.rs

@@ -100,6 +100,10 @@ fn parse_cfg_name_directive<'a>(
         name: "test",
         message: "always"
     }
+    condition! {
+        name: "auxiliary",
+        message: "used by another main test file"
+    }
     condition! {
         name: &config.target,
         allowed_names: &target_cfgs.all_targets,

src/tools/compiletest/src/header/tests.rs

@@ -940,3 +940,9 @@ fn test_supported_crate_types() {
         "//@ needs-crate-type: bin, cdylib, dylib, lib, proc-macro, rlib, staticlib"
     ));
 }
+
+#[test]
+fn test_ignore_auxiliary() {
+    let config = cfg().build();
+    assert!(check_ignore(&config, "//@ ignore-auxiliary"));
+}