test: add test case

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---

Author: headlessNode

lib/node_modules/@stdlib/ndarray/concat/test/test.assign.js

@@ -115,6 +115,45 @@ tape( 'the function throws an error if provided a second argument which is not a
 	}
 });
 
+tape( 'the function throws an error if provided a third argument which is not a negative integer', function test( t ) {
+	var values;
+	var x;
+	var y;
+	var z;
+	var i;
+
+	x = zeros( [ 2, 2 ] );
+	y = zeros( [ 2, 2 ] );
+	z = zeros( [ 2, 4 ] );
+
+	values = [
+		'5',
+		5.5,
+		0,
+		1,
+		2,
+		NaN,
+		true,
+		false,
+		null,
+		void 0,
+		[],
+		{},
+		function noop() {}
+	];
+
+	for ( i = 0; i < values.length; i++ ) {
+		t.throws( badValue( values[ i ] ), TypeError, 'throws an error when provided ' + values[ i ] );
+	}
+	t.end();
+
+	function badValue( value ) {
+		return function badValue() {
+			assign( [ x, y ], z, value );
+		};
+	}
+});
+
 tape( 'the function concatenates ndarrays along a specified dimension', function test( t ) {
 	var expected;
 	var actual;