Fix up potential race condition in test cases of "box" (fix #3759)

[seaoak]

What does it do?

Fix up Issue #3759

When "npm test" runs multiple times, a test case in test/scripts/box/box.js fails occasionally.
This is caused by a potential race condition.

The test case "process() - hash changed" fails
if it starts and finishes within wall-clock timer tick (=1ms).

This can be explained by the implementation of "box.Cache" (lib/models/cache.js).
"box.Cache" makes decision about "cache hit/miss" based on "mtime" and "hash" of target file.
"box.Cache" says "cache miss" only when both "mtime" and "hash" are changed.

The test case expects that "box.Cache" says "cache miss" always when "hash" is changed.
Unfortunately "box.Cache" says "cache hit" if "mtime" is not changed.

This patch is to clarify and complement these test cases.

How to test

$ while npm test; do :; done

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage remained the same at 97.273% when pulling e353f46 on seaoak:bugfix/test-of-box-fails-on-race-condition into 6f6084c on hexojs:master.