[meta] Fix and re-enable tests skipped for random failures, round 2

Problem/Motivation

Steps to reproduce

Proposed resolution

1. Un-skip one method per issue and run the affected test 500x.
2. Queue 8 test runs of 1000x against MySQL or MariaDB environments.
3. Post a patch that un-skips the test without any attempted fix to get a baseline fail rate, and always test this patch at the same time as any proposed fix with the same environment.
4. Skip other methods in the test if it becomes necessary due to out-of-scope fails.

Remaining tasks

• #3547493: [random test failure] MediaLibraryTest::testButton
• #3397387: [random test failure] MediaLinkabilityTest sometimes fails
• #3514699: Add a tolerance to width and height in ImageDimensionsTest::testImageDimensions()
• #3393477: [random test failure] LocaleUpdateTest
• #3569225: [random test failure] EntityReferenceWidgetTest::testWidgetPreview
• #3390881: [random test failure] Drupal.Tests.media_library.FunctionalJavascript.FieldUiIntegrationTest
• #3542883: [random test failure] Drupal\Tests\settings_tray\FunctionalJavascript\SettingsTrayBlockFormTest::testBlocks
• #3559550: [random test failure] UpdateSemverCoreSecurityCoverageTest
• #3396536: [random test failure] Re-enable AjaxTest::testAjaxFocus()
• #3150040: [random test failure] ScaffoldUpgradeTest and ScaffoldTest rely on packagist.org