Add test showing that private libs with same names don't get built

2 private libs with equivalent names but living in public names will not be
built as part of @install even if they have different public names

test/blackbox-tests/jbuild

@@ -297,3 +297,13 @@
      (progn
       (run ${exe:cram.exe} run.t)
       (diff? run.t run.t.corrected)))))))
+
+(alias
+ ((name runtest)
+  (deps ((files_recursively_in test-cases/installable-dup-private-libs)))
+  (action
+   (chdir test-cases/installable-dup-private-libs
+    (setenv JBUILDER ${bin:jbuilder}
+     (progn
+      (run ${exe:cram.exe} run.t)
+      (diff? run.t run.t.corrected)))))))

test/blackbox-tests/test-cases/installable-dup-private-libs/a1/jbuild

@@ -0,0 +1,3 @@
+(library
+ ((name a)
+  (public_name a1)))

test/blackbox-tests/test-cases/installable-dup-private-libs/a2/jbuild

@@ -0,0 +1,3 @@
+(library
+ ((name a)
+  (public_name a2)))

test/blackbox-tests/test-cases/installable-dup-private-libs/run.t

@@ -0,0 +1,7 @@
+  $ $JBUILDER build @install -j1 --display short --root .
+      ocamldep a2/a.ml.d
+        ocamlc a2/.a.objs/a.{cmi,cmo,cmt}
+      ocamlopt a2/.a.objs/a.{cmx,o}
+        ocamlc a2/a.cma
+      ocamlopt a2/a.{a,cmxa}
+      ocamlopt a2/a.cmxs