Configure test execution for the build_helper crate in bootstrap

To enable the previously moved `DropBomb` tests.

src/bootstrap/src/core/build_steps/test.rs

@@ -1352,6 +1352,53 @@ impl Step for CrateRunMakeSupport {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct CrateBuildHelper {
+    host: TargetSelection,
+}
+
+impl Step for CrateBuildHelper {
+    type Output = ();
+    const ONLY_HOSTS: bool = true;
+
+    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        run.path("src/tools/build_helper")
+    }
+
+    fn make_run(run: RunConfig<'_>) {
+        run.builder.ensure(CrateBuildHelper { host: run.target });
+    }
+
+    /// Runs `cargo test` for build_helper.
+    fn run(self, builder: &Builder<'_>) {
+        let host = self.host;
+        let compiler = builder.compiler(builder.top_stage, host);
+
+        builder.ensure(compile::Std::new(compiler, host));
+        let mut cargo = tool::prepare_tool_cargo(
+            builder,
+            compiler,
+            Mode::ToolStd,
+            host,
+            "test",
+            "src/tools/build_helper",
+            SourceType::InTree,
+            &[],
+        );
+        cargo.allow_features("test");
+        run_cargo_test(
+            cargo,
+            &[],
+            &[],
+            "build_helper",
+            "build_helper self test",
+            compiler,
+            host,
+            builder,
+        );
+    }
+}
+
 default_test!(Ui { path: "tests/ui", mode: "ui", suite: "ui" });
 
 default_test!(Crashes { path: "tests/crashes", mode: "crashes", suite: "crashes" });

src/bootstrap/src/core/builder.rs

@@ -860,6 +860,7 @@ impl<'a> Builder<'a> {
                 test::Clippy,
                 test::CompiletestTest,
                 test::CrateRunMakeSupport,
+                test::CrateBuildHelper,
                 test::RustdocJSStd,
                 test::RustdocJSNotStd,
                 test::RustdocGUI,