Fix Flaky test

[bwplotka]

reloader_test.go:269:

	exp: 5

	got: 3

--- FAIL: TestReloader_RuleApply (10.04s)
    reloader_test.go:238: Performing step number 1
    reloader_test.go:238: Performing step number 3
FAIL
FAIL	github.com/thanos-io/thanos/pkg/reloader	10.769s

Quite often spotted.

[johncming]

thanos/pkg/reloader/reloader_test.go

Line 182 in c35d47b

reloads.Store(reloads.Load().(int) + 1) // The only writer.

Reason: plus op is not an atomic operation.

[jojohappy]

@bwplotka @johncming I guess that the reloader read the middle status of watched files, because of using ioutil.WriteFile.

// WriteFile writes data to a file named by filename.
// If the file does not exist, WriteFile creates it with permissions perm;
// otherwise WriteFile truncates it before writing.

Then trigger reload handler twice, the atomic would be added twice.

[bwplotka]

@johncming I think this part is ok as there is only a single writer.

@jojohappy you are right but this can happen normally as well so we should handle it properly.

IMO it is just a CI slowdown, so increasing timeout here #1442 (: EDIT: Actually here: #1450