CI: restore input for test Langmuir_multi_nodal

[EZoni]

While running a number of CI tests through run_test.sh, I noticed that the following warning message appears, regardless of which tests are being executed:

finding tests and checking parameters...
WARNING: required params for test Langmuir_multi_nodal not set
         buildDir = .
         inputFile = 
         dim = 3
WARNING: test Langmuir_multi_nodal will be skipped

I found that the inputFile parameter for the test Langmuir_multi_nodal had been removed from WarpX-tests.ini in #4914, resulting in this test being skipped since then.

However, the test itself was kept in WarpX-tests.ini and given the description of #4914, seemingly not related to CI, I suspect that the removal of its input file might have been unintentional.

Assuming that that's the case, I'm restoring the input file for the test Langmuir_multi_nodal, allowing its execution through run_test.sh.

If instead the intention was in fact to skip the test permanently, please let me know and I will remove the corresponding entry from WarpX-tests.ini, along with all related files (e.g., JSON benchmark).