Closes #6172 - Convert waitNotNull into extension function for UI tests

[AaronMT]

#6172

From previous re-factor

It occurs to me that we could make this easier to reuse with an extension function. Something like the following:

    /**
     * Blocks the test for [waitTime] miliseconds before continuing.
     *
     * Will cause the test to fail is the condition is not met before the timeout.
     */
    fun UiDevice.waitNotNull(
        searchCondition: SearchCondition<*>,
        waitTime: Long = TestAssetHelper.waitingTimeShort
    ) = assertNotNull(wait(searchCondition, waitTime))

Which would update L113 to look like mDevice.waitNotNull(Until.findObjects(By.text("Sign in"))), or (if we wanted to use a different timeout) mDevice.waitNotNull(Until.findObjects(By.text("Sign in")), TestAssetHelper.waitingTimeShort)

[codecov-io]

Codecov Report

Merging #6220 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6220      +/-   ##
============================================
- Coverage     14.73%   14.69%   -0.04%     
  Complexity      319      319              
============================================
  Files           262      262              
  Lines         10732    10723       -9     
  Branches       1560     1558       -2     
============================================
- Hits           1581     1576       -5     
+ Misses         9025     9021       -4     
  Partials        126      126

Impacted Files	Coverage Δ	Complexity Δ
...ava/org/mozilla/fenix/browser/UriOpenedObserver.kt	47.05% <0%> (-12.04%)	3% <0%> (ø)
...a/org/mozilla/fenix/browser/BaseBrowserFragment.kt	0% <0%> (ø)	0% <0%> (ø)	⬇️

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[severinrudie]

LGTM! But I'm not on the test automation team, so I'll let someone else approve.