[gigasecond] Add a test to check that input is not mutated

exercises/gigasecond/.meta/hints.md

@@ -0,0 +1 @@
+It is possible to return a correct value for this exercise by mutating the solution function argument. Although there are legitimate use cases for mutating function arguments, this is usually undesirable, and in the case of this exercise, clearly unexpected. For this reason, the test suite has a test that fails in case the argument has been modified after the function execution.

exercises/gigasecond/README.md

@@ -5,6 +5,9 @@ has passed.
 
 A gigasecond is 10^9 (1,000,000,000) seconds.
 
+It is possible to return a correct value for this exercise by mutating the solution function argument. Although there are legitimate use cases for mutating function arguments, this is usually undesirable, and in the case of this exercise, clearly unexpected. For this reason, the test suite has a test that fails in case the argument has been modified after the function execution.
+
+
 ## Setup
 
 Go through the setup instructions for Javascript to install the necessary

exercises/gigasecond/gigasecond.spec.js

@@ -35,4 +35,10 @@ describe('Gigasecond', () => {
     const expectedDate = new Date(Date.parse('2046-10-03T01:46:39Z'));
     expect(gs).toEqual(expectedDate);
   });
+
+  xtest('does not mutate the input', () => {
+    const input = new Date(Date.UTC(2020, 0, 4, 20, 28, 30));
+    gigasecond(input);
+    expect(input).toEqual(new Date(Date.UTC(2020, 0, 4, 20, 28, 30)));
+  });
 });