Use a random directory to run the tests

[jgsogo]

When two different jobs point to the same commit they fight for the same directory because they are using a directory that contains the commit:

conan_ci_jenkins/src/org/jfrog/conanci/TestRunner.groovy

Line 166 in 7a237c4

base_source = "${base_dir}source/${commit}"

This scenario happens, for example, when two branches point to the same commit: master and release/1.xx and the symptom is a permission error in the CI (Windows) because one job is trying to override the python.exe that the other job is using:

PermissionError: [Errno 13] Permission denied: 'D:\\J\\t\\c5dd\\py38\\enabled_revisions\\venv\\Scripts\\python.exe'