I do not repro on master. I simplified by using an explicit target instead of changed to confirm I had a test actually run - which is needed to get any xml output:

$ ./pants test --test-pytest-junit-xml-dir=./tests-out/ tests/python/pants_test/util:memo
$ xmllint -format tests-out/TEST-tests.python.pants_test.util.memo.xml | head
<?xml version="1.0" encoding="utf-8"?>
<testsuite errors="0" failures="0" name="pytest" skips="0" tests="18" time="0.061">
  <testcase classname="...tests.python.pants_test.util.test_memo" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants/util/memo.py" line="258" name="testable_memoized_property" time="0.000488042831421"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="92" name="test_cache_factory" time="0.000428915023804"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="136" name="test_clear" time="0.000380039215088"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="216" name="test_descriptor_application_invalid" time="0.000358104705811"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="280" name="test_descriptor_application_valid" time="0.000385046005249"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="114" name="test_forget" time="0.000392913818359"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="29" name="test_function_application_kwargs" time="0.000377893447876"/>
  <testcase classname="...tests.python.pants_test.util.test_memo.MemoizeTest" file="pyprep/sources/15408d347b4dde741ac5a7c56a9e8f5fd927e173/pants_test/util/test_memo.py" line="43" name="test_function_application_mixed" time="0.000340938568115"/>

Happy to dig further and check against your Pants versions if you file an issue after confirming there aren't simply no tests running.