[BUGFIX] Give extension key to addPlugin() in registerPlugin()

With patch #59770 ExtensionManagementUtility::addPlugin() signature
was extended to get the extensionKey as parameter in order to not
use $GLOBALS['_EXTKEY'] anymore if possible.
The parameter is used to determine correct path to plugin icons if
the icon path is not given otherwise.
The patch changes extbase ExtensionUtility::registerPlugin() to give
the extensionKey to addPlugin, too.
With this patch registerPlugin() can be used in TCA/Overrides.
Some tests are added to verify the icon path calculation since it is
not trivial to follow the code flow with all its options and global
array access.

Change-Id: I2dd709ef7ca304bc8c4fda1e7911f18bfe7ad04c
Resolves: #60109
Related: #59770
Related: #59468
Related: #59414
Releases: 6.3, 6.2
Reviewed-on: https://review.typo3.org/31336
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters