Run test_2d_theta_implicit_strang_psatd test on one processor (#6050)

Author: dpgrote

Examples/Tests/implicit/CMakeLists.txt

@@ -75,7 +75,7 @@ if(WarpX_FFT)
     add_warpx_test(
         test_2d_theta_implicit_strang_psatd  # name
         2  # dims
-        2  # nprocs
+        1  # nprocs
         inputs_test_2d_theta_implicit_strang_psatd  # inputs
         "analysis_2d_psatd.py"  # analysis
         "analysis_default_regression.py --path diags/diag1000020"  # checksum