JuliaPackage: Allow running tests in a dedicated test_step

[Crivella]

Currently the only way to run the testsuite of a Julia package is to do so in the sanity_check_commands

This PR adds a way to use runtest and testcmd in the extension options to run the test suite of the package after installation.

To discuss

• Currently the test is ran allowing installation of online packages. This is done because tests can require extra dependencies that are not installed with the package.
  The alternative currently would be to install the test dependencies as normal one so that they are available to the test step. Alternatively we would need to add a new test_dep flag to Julia package so that the extension can be downloaded and added to the test_dir during test but would require altering the current logic a lot
• Possibly add a julia_cmd detection similar to python_cmd for PythonPackage

[Crivella]

Closing in favor of #4146 which includes these changes