fix install test at NERSC

[sbailey]

This PR fixes the desiutil installation test at NERSC. It was previously failing because it was picking /global/common/edison/contrib/desi/code/desiutil/master as the installation location, and then erroring because that already existed.

The test already included some logic to unset $DESI_PRODUCT_ROOT to try to avoid this, but that was unsuccessful because if that isn't set then the installation logic defaults to using /global/common/edison/contrib/desi/code anyway.

The workaround here is to test installing a fake branch "test-blat-foo". An alternate workaround would have been to set $DESI_PRODUCT_ROOT to some other location instead of unsetting it.

Note: I purposefully didn't update changes.rst since this seems too small to bother cluttering the human-friendly log of important changes to know about; i.e. let's keep the changes.rst focused on features and changes in functionality. @weaverba137 can override me as owner of this repo though.

For the record, this is what I get on edison when using desiutil master python setup.py test:

python setup.py test
...
======================================================================
ERROR: test_set_install_dir (desiutil.test.test_install.TestInstall)
Test the determination of the install directory.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/global/u2/s/sjbailey/temp/desiutil/py/desiutil/test/test_install.py", line 334, in test_set_install_dir
    install_dir = self.desiInstall.set_install_dir()
  File "/global/u2/s/sjbailey/temp/desiutil/py/desiutil/install.py", line 644, in set_install_dir
    raise DesiInstallException(message)
desiutil.install.DesiInstallException: Install directory, /global/common/edison/contrib/desi/code/desiutil/master, already exists!

----------------------------------------------------------------------
Ran 58 tests in 4.201s

FAILED (errors=1, skipped=1)

[weaverba137]

Thanks for catching this corner case. I don't see any major problems with this. Will merge after tests complete.