CRM_AllTests - Fix hidden GroupTest/TransactionTest interaction (#8115)

CRM_AllTests - Fix hidden GroupTest/TransactionTest interaction (#8115)

There's a hidden test interaction where CRM_Contact_BAO_GroupTest causes
CRM_Core_TransactionTest to fail, and this seems to involve munging the
active DB connection.

With this revision, CRM_Contact_BAO_GroupTest uses the same DB connection as
everything else.

The failure was reproducible using this command:

```
env CIVICRM_UF=UnitTests PHPUNIT_TESTS="CRM_Contact_BAO_GroupTest::testGroupData CRM_Core_TransactionTest::testBasicCommit" phpunit4  tests/phpunit/EnvTests.php
```