Add test case for missing argument value

While it's good to test around error conditions, the main purpose of this
test is to catch future changes in the error type and message.

Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

test/test_parse_args.cpp

@@ -3,6 +3,14 @@
 
 using doctest::test_suite;
 
+TEST_CASE("Missing argument" * test_suite("parse_args")) {
+  argparse::ArgumentParser program("test");
+  program.add_argument("--config").nargs(1);
+  REQUIRE_THROWS_WITH_AS(program.parse_args({ "test", "--config" }),
+                         "Too few arguments",
+                         std::runtime_error);
+}
+
 TEST_CASE("Parse a string argument with value" * test_suite("parse_args")) {
   argparse::ArgumentParser program("test");
   program.add_argument("--config");