software/theia/test: fix a flaky test

This test checks that running slapos node software command on theia shell starts installing software, but this command might get an error like:

process started, but another slapos process is aleady running with pid 2656158, exiting

this happened for example with https://erp5js.nexedi.net/#/test_result_module/20240911-ED8B4BF9/60