ok, some update:

1st: running the complete test suite with xvfb fails (as already reported):
> LD_LIBRARY_PATH="${srcdir}/Python-${pkgver}":${LD_LIBRARY_PATH} \
    xvfb-run "${srcdir}/Python-${pkgver}/python" -m test.regrtest -v -uall
...
0:02:14 load avg: 0.90 [174/403/1] test_idle
test_close (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_misc (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_read (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_readline (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_readlines (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_unsupported (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_close (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_misc (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_unsupported (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_write (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_writelines (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
/usr/bin/xvfb-run: line 181: 13491 Segmentation fault      (core dumped) DISPLAY=:$SERVERNUM XAUTHORITY=$AUTHFILE "$@" 2>&1


running w/o xvfb-run succeeds:
> LD_LIBRARY_PATH="${srcdir}/Python-${pkgver}":${LD_LIBRARY_PATH} ${srcdir}/Python-${pkgver}/python -m test.regrtest -v -m test_io test_idle
== CPython 2.7.15 (default, Jul 16 2018, 12:22:58) [GCC 8.1.1 20180531]
==   Linux-4.17.4-1.0-ARCH-i686-with-glibc2.1.3 little-endian
==   /home/vagrant/python2/src/Python-2.7.15/build/test_python_14208
== CPU count: 2
Run tests sequentially
0:00:00 load avg: 0.26 [1/1] test_idle
test_close (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_misc (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_read (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_readline (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_readlines (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_unsupported (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_close (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_misc (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_unsupported (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_write (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_writelines (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok

----------------------------------------------------------------------
Ran 11 tests in 0.003s

OK
1 test OK.

Total duration: 150 ms
Tests result: SUCCESS


running only these tests with xvfb-run _succeeds_:
> LD_LIBRARY_PATH="${srcdir}/Python-${pkgver}":${LD_LIBRARY_PATH} xvfb-run ${srcdir}/Python-${pkgver}/python -m test.regrtest -v -m test_io test_idle
== CPython 2.7.15 (default, Jul 16 2018, 12:22:58) [GCC 8.1.1 20180531]
==   Linux-4.17.4-1.0-ARCH-i686-with-glibc2.1.3 little-endian
==   /home/vagrant/python2/src/Python-2.7.15/build/test_python_14226
== CPU count: 2
Run tests sequentially
0:00:00 load avg: 0.15 [1/1] test_idle
test_close (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_misc (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_read (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_readline (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_readlines (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_unsupported (idlelib.idle_test.test_io.PseudeInputFilesTest) ... ok
test_close (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_misc (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_unsupported (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_write (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok
test_writelines (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... ok

----------------------------------------------------------------------
Ran 11 tests in 0.004s

OK
1 test OK.

Total duration: 144 ms
Tests result: SUCCESS