Add regression test for issue 18632 (#20308)

Closes #18632

It seems like the test is passing, probably due to changes with -Werror.

Author: sjrd

tests/neg/i18632.check

@@ -0,0 +1,5 @@
+-- [E176] Potential Issue Warning: tests/neg/i18632.scala:12:2 ---------------------------------------------------------
+12 |  bar // warn
+   |  ^^^
+   |  unused value of type String
+No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/i18632.scala

@@ -0,0 +1,14 @@
+//> using options -Wnonunit-statement -Werror
+
+class Context
+
+object Foo {
+  def run(program: Context ?=> String): Unit = ???
+}
+
+def bar(using Context): String = ???
+
+@main def run = Foo.run:
+  bar // warn
+  bar
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)