Split update script missing extension test to its own class

Problem/Motivation

256.274s Drupal\Tests\system\Functional\UpdateSystem\UpdateScriptTest             16 passed

---- Drupal\Tests\system\Functional\UpdateSystem\UpdateScriptTest ----
Status      Duration Info                                                                               
--------------------------------------------------------------------------------------------------------
Pass         10.266s testUpdateAccess                                                                
Pass         20.144s testRequirements                                                                
Pass         26.206s …ibilityChange with data set "module: core_version_requirement key incompatible"
Pass         14.281s …tibilityChange with data set "theme: core_version_requirement key incompatible"
Pass         26.138s testExtensionCompatibilityChange with data set "module: php requirement"        
Pass         14.306s testExtensionCompatibilityChange with data set "theme: php requirement"         
Pass         30.753s testMissingExtension with data set "core only"                                  
Pass         28.677s testMissingExtension with data set "contrib only"                               
Pass         51.869s testMissingExtension with data set "core and contrib"                           
Pass          4.315s testOrphanedSchemaEntries                                                       
Pass          2.699s testThemeSystem                                                                 
Pass          4.289s testNoUpdateFunctionality                                                       
Pass          6.169s testSuccessfulUpdateFunctionality                                               
Pass          4.000s testMaintenanceModeUpdateFunctionality                                          
Pass          4.347s testSuccessfulMultilingualUpdateFunctionality                                   
Pass          2.965s testMaintenanceModeLink     

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet