Update ThetaImplicitPicard_1d,SemiImplicitPicard_1d

AMReX default change: `amr.max_grid_size` is 64 from 32 now.

Since these tests are run with 2 MPI ranks, ensure we have at least
one box per MPI rank.

Regression/WarpX-tests.ini

@@ -3824,7 +3824,7 @@ analysisRoutine = Examples/Tests/point_of_contact_EB/analysis.py
 [ThetaImplicitPicard_1d]
 buildDir = .
 inputFile = Examples/Tests/Implicit/inputs_1d
-runtime_params = warpx.abort_on_warning_threshold=high
+runtime_params = warpx.abort_on_warning_threshold=high amr.max_grid_size=20
 dim = 1
 addToCompileString =
 cmakeSetupOpts = -DWarpX_DIMS=1
@@ -3868,7 +3868,7 @@ analysisRoutine = Examples/Tests/Implicit/analysis_vandb_jfnk_2d.py
 [SemiImplicitPicard_1d]
 buildDir = .
 inputFile = Examples/Tests/Implicit/inputs_1d_semiimplicit
-runtime_params = warpx.abort_on_warning_threshold=high
+runtime_params = warpx.abort_on_warning_threshold=high amr.max_grid_size=20
 dim = 1
 addToCompileString =
 cmakeSetupOpts = -DWarpX_DIMS=1