Add tests Path.descendent

Some of them are failing because it it doesn't work when the paths are equal or
have the same length

test/unit-tests/jbuild

@@ -37,3 +37,10 @@
          (glob_files ${SCOPE_ROOT}/src/*.cmi)
          (glob_files ${SCOPE_ROOT}/vendor/re/*.cmi)))
   (action (chdir ${SCOPE_ROOT} (run ${exe:expect_test.exe} ${<})))))
+
+(alias
+ ((name runtest)
+  (deps (path.mlt
+         (glob_files ${SCOPE_ROOT}/src/*.cmi)
+         (glob_files ${SCOPE_ROOT}/vendor/re/*.cmi)))
+  (action (chdir ${SCOPE_ROOT} (run ${exe:expect_test.exe} ${<})))))

test/unit-tests/path.mlt

@@ -0,0 +1,64 @@
+(* -*- tuareg -*- *)
+
+open Jbuilder;;
+open Import;;
+
+let r = Path.(relative root);;
+
+#install_printer Path.pp;;
+
+Path.(let p = relative root "foo" in descendant p ~of_:p)
+[%%expect{|
+val r : string -> Jbuilder.Path.t = <fun>
+Exception: Invalid_argument "String.sub / Bytes.sub".
+Raised at file "pervasives.ml", line 33, characters 25-45
+Called from file "string.ml", line 47, characters 2-23
+Called from file "src/path.ml", line 193, characters 13-71
+Called from file "toplevel/toploop.ml", line 180, characters 17-56
+|}]
+
+(* different strings but same length *)
+Path.(descendant (relative root "foo") ~of_:(relative root "bar"))
+[%%expect{|
+Exception: Invalid_argument "index out of bounds".
+Raised by primitive operation at file "src/path.ml", line 192, characters 29-39
+Called from file "toplevel/toploop.ml", line 180, characters 17-56
+|}]
+
+Path.(descendant (r "foo") ~of_:(r "foo/"))
+[%%expect{|
+Exception: Invalid_argument "String.sub / Bytes.sub".
+Raised at file "pervasives.ml", line 33, characters 25-45
+Called from file "string.ml", line 47, characters 2-23
+Called from file "src/path.ml", line 193, characters 13-71
+Called from file "toplevel/toploop.ml", line 180, characters 17-56
+|}]
+
+Path.(descendant (r "foo/") ~of_:(r "foo"))
+[%%expect{|
+Exception: Invalid_argument "String.sub / Bytes.sub".
+Raised at file "pervasives.ml", line 33, characters 25-45
+Called from file "string.ml", line 47, characters 2-23
+Called from file "src/path.ml", line 193, characters 13-71
+Called from file "toplevel/toploop.ml", line 180, characters 17-56
+|}]
+
+Path.(descendant (r "foo/bar") ~of_:(r "foo"))
+[%%expect{|
+- : Jbuilder.Path.t option = Some bar
+|}]
+
+Path.(descendant Path.root ~of_:(r "foo"))
+[%%expect{|
+- : Jbuilder.Path.t option = None
+|}]
+
+Path.(descendant Path.root ~of_:Path.root)
+[%%expect{|
+- : Jbuilder.Path.t option = Some 
+|}]
+
+Path.(descendant (r "foo") ~of_:Path.root)
+[%%expect{|
+- : Jbuilder.Path.t option = Some foo
+|}]