Fix error in test of uuid5 service

[thierrydallacroce]

1. Address the 2 errors below, returned by PHPUnit on our Uuid5 service.
2. Increase test coverage to include Uuid5's isValid().

1) Drupal\dkan_data\Tests\Unit\Uuid5Test::testGenerate with data set "string" ('foo', 'bar', 'fd088d96-7c6b-5adf-8581-cbb18a5dad67')
Non-static method Drupal\dkan_data\Service\Uuid5::generate() should not be called statically

/Users/thierry.dallacroce/Code/pqdc/docroot/profiles/contrib/dkan2/modules/custom/dkan_data/tests/src/Unit/Service/Uuid5Test.php:33

2) Drupal\dkan_data\Tests\Unit\Uuid5Test::testGenerate with data set "non-string" ('foo', stdClass Object (...), 'e9de513e-b4d7-5b05-902b-f90ee7a5db52')
Non-static method Drupal\dkan_data\Service\Uuid5::generate() should not be called statically

/Users/thierry.dallacroce/Code/pqdc/docroot/profiles/contrib/dkan2/modules/custom/dkan_data/tests/src/Unit/Service/Uuid5Test.php:33

ERRORS!
Tests: 2, Assertions: 0, Errors: 2.