Checkpoint-Restart Tests: 5 Digits

`regtest.py` in `regression_testing` hard-codes 5 digits:
```diff
--- a/regtest.py
+++ b/regtest.py
@@ -734,7 +734,7 @@ def test_suite(argv):
                 shutil.move(test.diffDir, orig_diff_dir)

             # get the file number to restart from
-            restart_file = "%s_chk%5.5d" % (test.name, test.restartFileNum)
+            restart_file = "%s_chk%6.6d" % (test.name, test.restartFileNum)
```

Regression/WarpX-tests.ini

@@ -71,7 +71,7 @@ cmakeSetupOpts = -DAMReX_ASSERTIONS=ON -DAMReX_TESTING=ON
 [pml_x_yee]
 buildDir = .
 inputFile = Examples/Tests/PML/inputs_2d
-runtime_params = warpx.do_dynamic_scheduling=0 algo.maxwell_solver=yee chk.file_prefix=pml_x_yee_chk
+runtime_params = warpx.do_dynamic_scheduling=0 algo.maxwell_solver=yee chk.file_prefix=pml_x_yee_chk chk.file_min_digits=5
 dim = 2
 addToCompileString =
 cmakeSetupOpts = -DWarpX_DIMS=2
@@ -104,7 +104,7 @@ analysisRoutine = Examples/Tests/PML/analysis_pml_ckc.py
 [pml_x_psatd]
 buildDir = .
 inputFile = Examples/Tests/PML/inputs_2d
-runtime_params = algo.maxwell_solver=psatd psatd.update_with_rho=1 warpx.do_dynamic_scheduling=0 diag1.fields_to_plot = Ex Ey Ez Bx By Bz rho divE warpx.cfl = 0.7071067811865475 warpx.do_pml_dive_cleaning=0 warpx.do_pml_divb_cleaning=0 chk.file_prefix=pml_x_psatd_chk
+runtime_params = algo.maxwell_solver=psatd psatd.update_with_rho=1 warpx.do_dynamic_scheduling=0 diag1.fields_to_plot = Ex Ey Ez Bx By Bz rho divE warpx.cfl = 0.7071067811865475 warpx.do_pml_dive_cleaning=0 warpx.do_pml_divb_cleaning=0 chk.file_prefix=pml_x_psatd_chk chk.file_min_digits=5
 dim = 2
 addToCompileString = USE_PSATD=TRUE
 cmakeSetupOpts = -DWarpX_DIMS=2 -DWarpX_PSATD=ON
@@ -1190,7 +1190,7 @@ analysisRoutine = Examples/analysis_default_regression.py
 [uniform_plasma_restart]
 buildDir = .
 inputFile = Examples/Physics_applications/uniform_plasma/inputs_3d
-runtime_params = chk.file_prefix=uniform_plasma_restart_chk
+runtime_params = chk.file_prefix=uniform_plasma_restart_chk chk.file_min_digits=5
 dim = 3
 addToCompileString =
 cmakeSetupOpts = -DWarpX_DIMS=3
@@ -1209,7 +1209,7 @@ analysisRoutine = Examples/Tests/restart/analysis_restart.py
 [restart]
 buildDir = .
 inputFile = Examples/Tests/restart/inputs
-runtime_params = chk.file_prefix=restart_chk
+runtime_params = chk.file_prefix=restart_chk chk.file_min_digits=5
 dim = 3
 addToCompileString =
 cmakeSetupOpts = -DWarpX_DIMS=3
@@ -1228,7 +1228,7 @@ analysisRoutine = Examples/Tests/restart/analysis_restart.py
 [restart_psatd]
 buildDir = .
 inputFile = Examples/Tests/restart/inputs
-runtime_params = algo.maxwell_solver=psatd psatd.use_default_v_galilean=1 particles.use_fdtd_nci_corr=0 chk.file_prefix=restart_psatd_chk boundary.field_lo=periodic periodic damped boundary.field_hi=periodic periodic damped
+runtime_params = algo.maxwell_solver=psatd psatd.use_default_v_galilean=1 particles.use_fdtd_nci_corr=0 chk.file_prefix=restart_psatd_chk chk.file_min_digits=5 boundary.field_lo=periodic periodic damped boundary.field_hi=periodic periodic damped
 dim = 3
 addToCompileString = USE_PSATD=TRUE
 cmakeSetupOpts = -DWarpX_DIMS=3 -DWarpX_PSATD=ON
@@ -1247,7 +1247,7 @@ analysisRoutine = Examples/Tests/restart/analysis_restart.py
 [restart_psatd_time_avg]
 buildDir = .
 inputFile = Examples/Tests/restart/inputs
-runtime_params = algo.maxwell_solver=psatd psatd.use_default_v_galilean=1 particles.use_fdtd_nci_corr=0 chk.file_prefix=restart_psatd_time_avg_chk boundary.field_lo=periodic periodic damped boundary.field_hi=periodic periodic damped psatd.do_time_averaging=1
+runtime_params = algo.maxwell_solver=psatd psatd.use_default_v_galilean=1 particles.use_fdtd_nci_corr=0 chk.file_prefix=restart_psatd_time_avg_chk chk.file_min_digits=5 boundary.field_lo=periodic periodic damped boundary.field_hi=periodic periodic damped psatd.do_time_averaging=1
 dim = 3
 addToCompileString = USE_PSATD=TRUE
 cmakeSetupOpts = -DWarpX_DIMS=3 -DWarpX_PSATD=ON