Test case to reproduce scope bug when building ppx

The scope selected when building the ppx is bogus (it corresponds to the
directory with the ppx exe).

test/blackbox-tests/jbuild

@@ -267,3 +267,13 @@
      (progn
       (run ${exe:cram.exe} run.t)
       (diff? run.t run.t.corrected)))))))
+
+(alias
+ ((name runtest)
+  (deps ((files_recursively_in test-cases/scope-ppx-bug)))
+  (action
+   (chdir test-cases/scope-ppx-bug
+    (setenv JBUILDER ${bin:jbuilder}
+     (progn
+      (run ${exe:cram.exe} run.t)
+      (diff? run.t run.t.corrected)))))))

test/blackbox-tests/test-cases/scope-ppx-bug/a/kernel/jbuild

@@ -0,0 +1,6 @@
+(jbuild_version 1)
+
+(library
+ ((name a_kernel)
+  (public_name a.kernel)
+  (kind ppx_rewriter)))

test/blackbox-tests/test-cases/scope-ppx-bug/a/ppx/jbuild

@@ -0,0 +1,7 @@
+(jbuild_version 1)
+
+(library
+ ((name a)
+  (public_name a)
+  (kind ppx_rewriter)
+  (libraries (a_kernel))))

test/blackbox-tests/test-cases/scope-ppx-bug/b/jbuild

@@ -0,0 +1,6 @@
+(jbuild_version 1)
+
+(library
+ ((name b)
+  (public_name b)
+  (preprocess (pps (a)))))

test/blackbox-tests/test-cases/scope-ppx-bug/run.t

@@ -0,0 +1,5 @@
+  $ $JBUILDER build -j1 --root . @install
+  Error: External library "a_kernel" not found.
+  -> required by ".ppx/a_kernel/jbuild (context default)"
+  Hint: try: jbuilder external-lib-deps --missing --root . @install
+  [1]