Tests: Add test for exported JARs

Add a unit test to test the new functions to export JAR targets. The
test builds three sub-projects: two that generate and export a JAR (one
does a build-directory-only export, one an install-only export), and a
third that consumes the first two as imported targets.

Tests/JavaExportImport/main.c

@@ -0,0 +1,4 @@
+int main()
+{
+  return 0;
+}