suite.py: Avoid Duplicates in addToCompileString

[ax3l]

Potentially needs a backport from @EZoni in ECP-WarpX#13:

This should fix ECP-WarpX/WarpX#2068.

This PR can be tested by running

./run_test.sh Langmuir_multi_2d_nodal Langmuir_multi_2d_psatd

from a local WarpX installation, with and without the changes in this PR. With the changes in this PR, we avoid two duplicate builds for the two tests above, by removing the duplicate build option USE_PSATD=TRUE (otherwise appearing twice) from the final compile string.

Note that you will have to modify run_test.sh by commenting and replacing the following line

#git clone https://github.com/ECP-WarpX/regression_testing.git
cp -r /<path_to>/regression_testing .

in order to use your local clone of the regression testing repository (and thus be able to switch between the master branch and the branch of this PR).