Add test witnessing generation of backend dune files

It shows that jbuilder fails to correctly escape the action field

test/blackbox-tests/test-cases/inline_tests/dune-file/jbuild

@@ -0,0 +1,26 @@
+(library
+ ((name foo)
+  (public_name foo)
+  (modules ())
+  (inline_tests.backend
+   ((runner_libraries (ppx_inline_test.runner.lib))
+    (generate_runner
+     (progn
+      (echo "let () = print_int 41")
+      (echo "\n")
+      (echo "let () = print_int 42")
+      (echo "\n")
+      (echo "let () = print_int 43;;")))
+    (flags (inline-test-runner ${library-name}
+             -source-tree-root ${ROOT} -diff-cmd -))))))
+
+(library
+ ((name foo_tests)
+  (inline_tests ((backend foo)))))
+
+(alias
+ ((name runtest)
+  (deps (foo.dune))
+  (action (echo "${read:foo.dune}"))))
+
+(jbuild_version 1)

test/blackbox-tests/test-cases/inline_tests/run.t

@@ -19,3 +19,27 @@
   $ $JBUILDER runtest many-backends-choose -j1 --display quiet --root .
            run alias many-backends-choose/runtest
   backend_mbc1
+
+  $ $JBUILDER runtest dune-file -j1 --display quiet --root .
+  (dune
+   1
+   ((inline_tests.backend
+     1.0
+     ((runner_libraries (ppx_inline_test.runner.lib))
+      (flags
+       (inline-test-runner
+        ${library-name}
+        -source-tree-root
+        ${ROOT}
+        -diff-cmd
+        -))
+      (generate_runner
+       ((progn
+         (echo let () = print_int 41)
+         (echo "\n")
+         (echo let () = print_int 42)
+         (echo "\n")
+         (echo let () = print_int 43;;))))
+      (extends ())))))
+           run alias dune-file/runtest
+  414243