Improve handling of temporary directories in SQL tests.

Initialize directory delayed in shared code and add checks to verify
that it is valid. Close attached / cloned databases to prevent locks
on files and leaking temporary directories caused by SQLite:

QTemporaryDir: Unable to remove "...\Temp\tst_qsqldatabase-P1XkOA" most likely due to the presence of read-only files.
QTemporaryDir: Unable to remove "...\Temp\tst_qsqltablemodel-P1XkOA" most likely due to the presence of read-only files.
QWARN  : tst_QSql::concurrentAccess() QTemporaryDir: Unable to remove "...\Temp\tst_qsql-l0VAKJ" most likely due to the presence of read-only files.

In Qt 4, emulate the temporary directory by maintaining
a list of files and deleting them in reverse order.

Change-Id: If85bbaed04bb1a32e427d642be332996d967f796
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/9a4beb4d36dc0c613e59aa5d88060d521462f56c)