bashCompletion: speed-up test execution by using xdist

Currently the tests take an eternity and are also sometimes flaky. By
following upstream in using xdist for parallel test execution we at
least get the feedback cycle down. On my machine that means instead of
running this for ~25min it runs in 1 minute and 10 seconds.

pkgs/shells/bash/bash-completion/default.nix

@@ -30,6 +30,10 @@ stdenv.mkDerivation rec {
     python3Packages.pexpect
     python3Packages.pytest
     bashInteractive
+
+    # use xdist to speed up the test run, just like upstream:
+    # https://github.com/scop/bash-completion/blob/009bf2228c68894629eb6fd17b3dc0f1f6d67615/test/requirements.txt#L4
+    python3Packages.pytest_xdist
   ];
 
   # - ignore test_gcc on ARM because it assumes -march=native
@@ -40,7 +44,7 @@ stdenv.mkDerivation rec {
   # - ignore test_ls because impure logic
   # - ignore test_screen because it assumes vt terminals exist
   checkPhase = ''
-    pytest . \
+    pytest -n $NIX_BUILD_CORES . \
       ${stdenv.lib.optionalString (stdenv.hostPlatform.isAarch64 || stdenv.hostPlatform.isAarch32) "--ignore=test/t/test_gcc.py"} \
       --ignore=test/t/test_chsh.py \
       --ignore=test/t/test_ether_wake.py \