[test] Add experimental ChiselStage extensions

Add a utility that can be used to replace
`ChiselRunners.elaborateAndGetModule`.  This is intentionally private to
the `chiselTests.experimental` package as I don't want this method to be
used outside of some of the more exotic D/I or DataView APIs which want to
use this.  This also is not something that makes sense (right now) to
expose on `ChiselStage` publicly.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

src/test/scala-2/chiselTests/experimental/ExtensionMethods.scala

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chiselTests.experimental
+
+import chisel3.RawModule
+import circt.stage.ChiselStage
+
+/** Object that contains various extension methods for all the [[experimental]] package. */
+private[experimental] object ExtensionMethods {
+
+  /** Extension methods for [[ChiselStage]]. */
+  implicit class ChiselStageHelpers(obj: ChiselStage.type) {
+
+    /** Construct a module and return it.  This has a _very_ narrow use case and
+      * should not be generally used for writing tests of Chisel.
+      */
+    def getModule[A <: RawModule](gen: => A): A = {
+      var res: Any = null
+      obj.convert {
+        res = gen
+        res.asInstanceOf[A]
+      }
+      res.asInstanceOf[A]
+    }
+
+  }
+
+}