migraphx: Old to new test API (#44988)

* migraphx: Old to new test API
* migraphx: tweak name and docstring to be more descriptive

---------

Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.com>

var/spack/repos/builtin/packages/migraphx/package.py

@@ -136,10 +136,10 @@ def cmake_args(self):
             )
         return args
 
-    def test(self):
+    def test_unit_tests(self):
+        """Run installed UnitTests"""
         if self.spec.satisfies("@:5.5.0"):
-            print("Skipping: stand-alone tests")
-            return
-        test_dir = join_path(self.spec["migraphx"].prefix, "bin")
-        with working_dir(test_dir, create=True):
-            self.run_test("UnitTests")
+            raise SkipTest("Package must be installed as version @5.5.1 or later")
+        unit_tests = which(self.prefix.bin.UnitTests)
+        assert unit_tests is not None, "UnitTests is not installed!"
+        unit_tests()