Fix Oracle test suite

Final fixes for the Oracle test suite to run again without errors:

```bash
deeky@98N-MUELLER-STEVE:~/dev/doctrine/dbal$ phpunit -c oci8.phpunit.xml
PHPUnit 3.7.27 by Sebastian Bergmann.

Configuration read from /home/deeky/dev/doctrine/dbal/oci8.phpunit.xml

..........................................................SSS   61 / 1578 (  3%)
S.............................S.SS...SSSSSSSSSSSSSS.S.SSSSS..  122 / 1578 (  7%)
..SSS.....................SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS  183 / 1578 ( 11%)
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS..........S.......S  244 / 1578 ( 15%)
......SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS  305 / 1578 ( 19%)
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS  366 / 1578 ( 23%)
S..SSS..SS.S.........................S.......................  427 / 1578 ( 27%)
.............................................................  488 / 1578 ( 30%)
.............................................................  549 / 1578 ( 34%)
.............................................................  610 / 1578 ( 38%)
...........................................................S.  671 / 1578 ( 42%)
.............................................................  732 / 1578 ( 46%)
.....................................S......S................  793 / 1578 ( 50%)
.............................................................  854 / 1578 ( 54%)
......................S.....S................................  915 / 1578 ( 57%)
.............................................................  976 / 1578 ( 61%)
............................................................. 1037 / 1578 ( 65%)
............................................................. 1098 / 1578 ( 69%)
.......................................................SSS... 1159 / 1578 ( 73%)
............................................................. 1220 / 1578 ( 77%)
............................................................. 1281 / 1578 ( 81%)
............................................................. 1342 / 1578 ( 85%)
................S............................................ 1403 / 1578 ( 88%)
............................................................. 1464 / 1578 ( 92%)
.....S..SSS.................................................. 1525 / 1578 ( 96%)
.....................................................

Time: 2.19 minutes, Memory: 50.25Mb

OK, but incomplete or skipped tests!
Tests: 1578, Assertions: 2852, Skipped: 246.
```

**Changes**
- Skip tests for unsupported rollback of DDL statements during transactions
- Fix default NULL value (again) containing trailing whitespace when retrieved from database
- Fix drop and create database test by utilizing temporary connection in favour of real connection which has sufficient privileges and changing database name to match Oracle 12c requirements

And finally... HALLELUJAH! :+1: