test: add missing thisArg tests

Author: kgryte

lib/node_modules/@stdlib/array/base/group-indices-by/test/test.js

@@ -164,3 +164,30 @@ tape( 'the function returns an empty object if provided an empty array', functio
 		return v;
 	}
 });
+
+tape( 'the function supports specifying a callback execution context', function test( t ) {
+	var expected;
+	var out;
+	var ctx;
+	var x;
+
+	x = [ 'beep', 'boop', 'foo', 'bar' ];
+
+	expected = {
+		'b': [ 0, 1, 3 ],
+		'f': [ 2 ]
+	};
+	ctx = {
+		'count': 0
+	};
+	out = groupIndicesBy( x, indicator, ctx );
+
+	t.deepEqual( out, expected, 'returns expected value' );
+	t.strictEqual( ctx.count, x.length, 'returns expected value' );
+	t.end();
+
+	function indicator( v ) {
+		this.count += 1; // eslint-disable-line no-invalid-this
+		return v[ 0 ];
+	}
+});