Sorry, this is a bit involved :-(
BTW all the tests except this one pass (ctypes is disabled on x86_64 and arm64) on Android API 24 for x86, x86_64, armv7 and arm64 :-)

Description:
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There are two different cases:
1) When buildbottest is tweaked to run only test_regrtest and is run remotely from the build system, the error occurs systematically and is:

      Warning -- files was modified by test_regrtest
        Before: []
        After:  ['test_python_2535/']

Here is the command that is run remotely (through adb), the run_tests.py file is Tools/scripts/run_tests.py from the src tree that has been pushed to the emulator by buildbottest:

python /data/local/tmp/python/bin/run_tests.py -j 1 -u all -W --slowest --fail-env-changed --timeout=900 -j2 test_regrtest

The command also fails when replacing '-j2' with '-j1'.

In that case:
* There is no 'test_support_*' directory left over in the TEMP directory, the directory is clean.
* The command 'adb logcat' reports a crash ("Fatal signal 11 (SIGSEGV)") during the execution of the command. This sounds familiar :-)


2) When this same command is run on the emulator (i.e. the user is remotely logged in with the command 'adb shell'), the environment is never altered (never == about 20 attempts to raise the problem).

In that case:
* A 'test_support_*' directory is left in the TEMP directory and it is empty.

The fact that the TEMP directory is not clean already happens when running buildbottest natively with the standard Python Makefile (no one has noticed it yet I guess) and the directory contains a core file. This is another bug, related to this one and it provided a much welcome hint to use 'adb logcat' and for a work-around :-)
Maybe this last bug is related to issue 26295 ?

Work-around
-----------
* The problem does not happen when skipping ArgsTestCase.test_crashed for Android in the same manner that tests that raise SIGSEGV are skipped in test_faulthandler. And there is no 'test_support_*' directory left over in the TEMP directory.