Add unit tests for String.{take,drop,split_n}

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

test/unit-tests/dune

@@ -98,3 +98,13 @@
           (progn
            (run %{exe:expect_test.exe} %{t})
            (diff? %{t} %{t}.corrected)))))
+
+(alias
+ (name runtest)
+ (deps (:t string.mlt)
+  (glob_files %{project_root}/src/.dune.objs/*.cmi)
+  (glob_files %{project_root}/src/stdune/.stdune.objs/*.cmi))
+ (action (chdir %{project_root}
+          (progn
+           (run %{exe:expect_test.exe} %{t})
+           (diff? %{t} %{t}.corrected)))))

test/unit-tests/string.mlt

@@ -0,0 +1,89 @@
+(* -*- tuareg -*- *)
+open! Stdune;;
+
+Printexc.record_backtrace false;;
+
+String.take "foobar" 3;;
+[%%expect{|
+- : unit = ()
+- : string = "foo"
+|}]
+
+String.take "foobar" 0;;
+[%%expect{|
+- : string = ""
+|}]
+
+String.take "foo" 10;;
+[%%expect{|
+- : string = "foo"
+|}]
+
+String.take "" 10;;
+[%%expect{|
+- : string = ""
+|}]
+
+String.take "" 0;;
+[%%expect{|
+- : string = ""
+|}]
+
+
+String.drop "" 0;;
+[%%expect{|
+- : string = ""
+|}]
+
+String.drop "foo" 0;;
+[%%expect{|
+- : string = "foo"
+|}]
+
+String.drop "foo" 5;;
+[%%expect{|
+- : string = ""
+|}]
+
+String.drop "foobar" 3;;
+[%%expect{|
+- : string = "bar"
+|}]
+
+String.split_n "foobar" 3;;
+[%%expect{|
+- : string * string = ("foo", "bar")
+|}]
+
+String.split_n "foobar" 10;;
+[%%expect{|
+Exception:
+Code_error
+ (List
+   [Atom "String.split_n"; List [Atom "s"; Atom "foobar"];
+    List [Atom "n"; Atom "10"]]).
+|}]
+
+String.split_n "foobar" 0;;
+[%%expect{|
+- : string * string = ("", "foobar")
+|}]
+
+String.split_n "foobar" 6;;
+[%%expect{|
+- : string * string = ("foobar", "")
+|}]
+
+String.split_n "" 0;;
+[%%expect{|
+- : string * string = ("", "")
+|}]
+
+String.split_n "" 10;;
+[%%expect{|
+Exception:
+Code_error
+ (List
+   [Atom "String.split_n"; List [Atom "s"; Atom ""];
+    List [Atom "n"; Atom "10"]]).
+|}]