nexus qdens test is failing on some systems

**Describe the bug**

https://cdash.qmcpack.org/CDash/testSummary.php?project=1&name=ntest_nexus_qdens&date=2021-04-10

Listed as failing with gcc10 and pgi on nitrogen, also failing with gcc10 on sulfur. However also working with Intel2019.1 on sulfur and other system. Seems most likely to be a configuration issue+more error trapping needed, but the error message is cryptic. Ideas?

```
Start 1557: ntest_nexus_qdens

1557: Test command: /qmcpack/nexus/bin/nxs-test "-R" "qdens" "--ctest" "--pythonpath=git_QMCPACK_prckent/qmcpack/nexus/lib"
1557: Test timeout computed to be: 3600
1557: Test name : qdens
1557: Test sublabel : test_density
1557: Test exception: "AssertionError: "
1557: Test backtrace:
1557: File "git_QMCPACK_prckent/qmcpack/nexus/bin/nxs-test", line 478, in run
1557: self.operation()
1557: File "git_QMCPACK_prckent/qmcpack/nexus/bin/nxs-test", line 1626, in qdens
1557: nunit_all()
1557: File "git_QMCPACK_prckent/qmcpack/nexus/bin/nxs-test", line 362, in nunit_all
1557: run_external_unit_test(test_name,unit_test)
1557: File "git_QMCPACK_prckent/qmcpack/nexus/bin/nxs-test", line 388, in run_external_unit_test
1557: unit_test()
1557: File "git_QMCPACK_prckent/qmcpack/nexus/tests/unit/test_qdens.py", line 68, in test_density
1557: assert(check_value_eq(set(os.listdir(dmc_path)),set(files_aft)))
1557:
1557: Test status: fail
1/1 Test #1557: ntest_nexus_qdens ................***Failed 0.59 sec
```
**To Reproduce**

ctest -R ntest_nexus_qdens

**Expected behavior**

Test passes

**System:**

As described.

**Additional context**