fix(discovery tests): allow requireUpdate util to timeout in discovery/file…

[machine424]

…/file_test.go.

The loop ran indefinitely if the condition isn't met.

Before, each iteration created a new timer channel which was always outpaced by the other timer channel with smaller duration.

minor detail: There was a memory leak: resources of the ~10 previous timers were constantly kept. With the fix, we may keep the resources of one timer around for defaultWait but this isn't worth the changes to make it right.

[machine424]

To reproduce the issue:

@@ -383,7 +382,7 @@ func TestFileUpdate(t *testing.T) {
        defer runner.stop()
 
        // Verify that we receive the initial target groups.
-       runner.requireUpdate(time.Time{}, validTg(sdFile))
+       runner.requireUpdate(time.Time{}, []*targetgroup.Group{})
 
        // Verify that we receive an update with the new target groups.

And run the test.
(I didn't think this is worth an issue, but tell me if I should create one)

[machine424]

cc @bboreham
Another easy one.

[bboreham]

Good catch, thanks.