Revert "bashCompletion: speed-up test execution by using xdist"

This reverts commit a85b07cbcb as
executing the tests in parallel makes them flaky. This can be seen very
easily on armv7l machines (and probably other machines that are slower
than common x86_64 machines as well), but is also reproducible on
x86_64.

This fixes #91706.

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

@@ -30,10 +30,6 @@ 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
@@ -44,7 +40,7 @@ stdenv.mkDerivation rec {
   # - ignore test_ls because impure logic
   # - ignore test_screen because it assumes vt terminals exist
   checkPhase = ''
-    pytest -n $NIX_BUILD_CORES . \
+    pytest . \
       ${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 \