[webview_flutter_wkwebview] Fix potential timeouts in the integration tests

[bparrishMines]

Copy of fix for webview_flutter and webview_Flutter_android: #6857

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• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[cyanglaz]

Code LGTM, CI is failing on XCUITests and seems unrelated:

Test Case '-[FLTWebViewUITests testTransparentBackground]' started.
    t =     0.00s Start Test at 2022-12-20 10:24:22.169
    t =     0.06s Set Up
    t =     0.06s     Open dev.flutter.plugins.webviewFlutterExample
    t =     0.08s         Launch dev.flutter.plugins.webviewFlutterExample
    t =     0.29s             Setting up automation session
    t =     2.48s             Wait for dev.flutter.plugins.webviewFlutterExample to idle
    t =     4.01s Waiting 30.0s for "Show menu" Button to exist
    t =     5.09s     Checking `Expect predicate `exists == 1` for object "Show menu" Button`
    t =     5.09s         Checking existence of `"Show menu" Button`
    t =     5.16s Tap "Show menu" Button
    t =     5.16s     Wait for dev.flutter.plugins.webviewFlutterExample to idle
    t =     6.38s     Find the "Show menu" Button
    t =     6.41s     Check for interrupting elements affecting "Show menu" Button
    t =     6.44s     Synthesize event
    t =     6.85s     Wait for dev.flutter.plugins.webviewFlutterExample to idle
    t =     6.89s Waiting 30.0s for "Transparent background example" Button to exist
    t =     7.92s     Checking `Expect predicate `exists == 1` for object "Transparent background example" Button`
    t =     7.92s         Checking existence of `"Transparent background example" Button`
    t =     7.96s Tap "Transparent background example" Button
    t =     7.96s     Wait for dev.flutter.plugins.webviewFlutterExample to idle
    t =     7.98s     Find the "Transparent background example" Button
    t =     8.04s     Check for interrupting elements affecting "Transparent background example" Button
    t =     8.08s     Synthesize event
    t =     8.39s     Wait for dev.flutter.plugins.webviewFlutterExample to idle
    t =     8.44s Waiting 30.0s for "Transparent background test" StaticText to exist
    t =     9.49s     Checking `Expect predicate `exists == 1` for object "Transparent background test" StaticText`
    t =     9.49s         Checking existence of `"Transparent background test" StaticText`
/Volumes/Work/s/w/ir/x/w/plugins/packages/webview_flutter/webview_flutter_wkwebview/example/ios/RunnerUITests/FLTWebViewUITests.m:85: error: -[FLTWebViewUITests testTransparentBackground] : ((CGColorEqualToColor(redColor, centerColor.CGColor)) is true) failed

[stuartmorgan-g]

@bparrishMines Does this just need the latest main merged in?

[bparrishMines]

Possibly. I wanted to explore the flake that @cyanglaz pointed out. But it could end up just being fixed after pulling from main. I'll try.

[bparrishMines]

Fixed in #6996