objectcache: Improve WANObjectCache test coverage

objectcache: Improve WANObjectCache test coverage

class WANObjectCache (methods: 30% -> 40%, lines: 62% -> 65%)

* doGetWithSetCallback: 98% -> 100%
 - Add case: Throw for invalid callback.
* getWithSetCallback: 86% -> 100%
 - Cover case: Else branch for `$cur[self::VFLD_VERSION] === $version`.
* getMulti: 91% -> 98%
 - Cover case: If branch for `$this->warmupCache`.
* set: 93% -> 100%
 - Cover case: If branch for `lockTSE >= 0`.
* reap: 84% -> 100%
 - Add case: Error when changeTTL() fails.

Change-Id: I4844ab0ebdd4d3ec9acc9cd4500721b04cfda317