[5.0] make resource monitor plugin tests deterministic by using fixed fc::temp_directory instead of dynamic /tmp

[linh2931]

Several resource monitor plugin tests use native /tmp as a monitored directory for testing. However, subdirectories in /tmp on test machine can be removed during a test run, causing file_space_handler::add_file_system to assert when doing get_stat on a removed directory, as shown by following failure:

/__w/leap/leap/plugins/resource_monitor_plugin/test/test_resmon_plugin.cpp(147): fatal error: in "resmon_plugin_tests/startupNormal": unexpected exception thrown by plugin_startup({"/tmp"})

I manually simulated a removed directory and reproduced the same failure.

This PR

• uses a fixed temp directory provided by fc::temp_directory instead of /tmp.
• removes a non-necessary no-value provided test.

Resolves #1824