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

And finally... HALLELUJAH! :+1: 
- 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