Add temporary workflow to check new backend (#308)

* Add temporary workflow to check new backend

* Try to temporarily disable yjit-ubuntu workflows

* Add new btest file for new backend tests

bootstraptest/test_yjit_new_backend.rb

@@ -0,0 +1,40 @@
+# Run can run this test file directly with:
+# make -j miniruby && RUST_BACKTRACE=1 ruby --disable=gems bootstraptest/runner.rb --ruby="./miniruby -I./lib -I. -I.ext/common --disable-gems --yjit-call-threshold=1 --yjit-verify-ctx" bootstraptest/test_yjit_new_backend.rb
+
+assert_equal '3', %q{
+    def foo(n)
+      n
+    end
+    foo(3)
+}
+
+assert_equal '14', %q{
+    def foo(n)
+      n + n
+    end
+    foo(7)
+}
+
+assert_equal '[7]', %q{
+    def foo(n)
+      [n]
+    end
+    foo(7)
+}
+
+assert_equal '[8]', %q{
+    def foo(n)
+      [n+1]
+    end
+    foo(7)
+}
+
+# TODO: progress towards getting branches and calls working
+=begin
+def foo(n)
+    if n
+        n
+    end
+end
+puts foo(0)
+=end