test: specify intended features of dune runtest

Signed-off-by: Ali Caglayan <alizter@gmail.com>

test/blackbox-tests/test-cases/runtest-cmd.t

@@ -0,0 +1,48 @@
+Here we test the features of the `dune runtest` command. 
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.16)
+  > EOF
+
+  $ cat > mytest.t <<EOF
+  >   $ echo "Hello, world!"
+  > "Goodbye, world!"
+  > EOF
+  $ mkdir -p tests/myothertest.t
+  $ cat > tests/myothertest.t/run.t <<EOF
+  >   $ echo "Hello, world!"
+  > "Goodbye, world!"
+  > EOF
+
+Passing no arguments to `dune runtest` should be equivalent to `dune build
+@runtest`.
+
+  $ dune test 2>&1 | grep "^File"
+  File "mytest.t", line 1, characters 0-0:
+  File "tests/myothertest.t/run.t", line 1, characters 0-0:
+
+Passing the name of a test should only run that test.
+Currently, this is not the case.
+
+  $ dune test mytest
+  Error: Don't know about directory mytest specified on the command line!
+  [1]
+  $ dune test mytest.t
+  Error: Don't know about directory mytest.t specified on the command line!
+  [1]
+  $ dune test tests/myothertest
+  Error: Don't know about directory tests/myothertest specified on the command
+  line!
+  [1]
+  $ dune test tests/myothertest.t
+
+Passing a directory should run all the tests in that directory (recursively).
+
+The current working directory:
+  $ dune test . 2>&1 | grep "^File"
+  File "mytest.t", line 1, characters 0-0:
+  File "tests/myothertest.t/run.t", line 1, characters 0-0:
+
+The tests/ subdirectory:
+  $ dune test tests/ 2>&1 | grep "^File"
+  File "tests/myothertest.t/run.t", line 1, characters 0-0: