Use a local temp directory in logging system tests

Previously, the tests for Boot’s various logging systems used the
JVM’s default temp directory as the location of the spring.log file.
It is suspected that this was causing intermittent CI failures when
multiple Boot builds were running in parallel and tests were checking
for the presence of the spring.log file in the shared temp directory.

This commit updates AbstractLoggingSystemTests to configure a local
temp directory for the duration of the tests, thereby hopefully
eliminating failures caused by concurrent builds sharing the same
directory.

The previous attempt at fixing the intermittent CI failures (504de8a)
has been removed as part of this commit as it did not fix the problem.

Closes gh-1864