Fix JavaMemoryAssistant spec missing test helper

Add missing sub_configuration_context helper method to
java_memory_assistant_spec.rb. This test has been broken since it was
originally added in commit f2eee85 (Oct 26, 2016).

The test references sub_configuration_context() on line 59 but never
defined the helper method. Other specs (tomcat_spec.rb,
open_jdk_like_spec.rb, ibm_jre_spec.rb) each define their own local
helper following the same pattern.

This is a pre-existing bug unrelated to the Ruby 3.4 migration - the
test would have failed on any Ruby version (2.5, 2.6, 2.7, 3.0, 3.1, 3.4)
with NoMethodError for undefined method 'sub_configuration_context'.

Fixes #436 (original PR where test was introduced)

Author: ramonskie

spec/java_buildpack/framework/java_memory_assistant_spec.rb

@@ -73,3 +73,9 @@ class StubJavaMemoryAssistant < JavaBuildpack::Framework::JavaMemoryAssistant
   public :command, :sub_components, :supports?
 
 end
+
+def sub_configuration_context(configuration)
+  c = context.clone
+  c[:configuration] = configuration
+  c
+end