[BUGFIX] Helper functions break connection to current database

Calling createDatabase(), cleanDatabase() or dropDatabase() breaks the
database connection which is defined in
$GLOBALS['TYPO3_CONF_VARS']['DB']['database‘]. This patch reconnects to
this database within these methods.

Change-Id: I72f9c14bea7b839e13472bd112fd887134f2e020
Resolves: #55144
Releases: 3.7, 3.8
Reviewed-on: https://review.typo3.org/26939
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee