permut package tests fail with the master branch

[olexandr-konovalov]

The following failures from https://travis-ci.org/gap-system/gap-docker-pkg-tests-master/jobs/331580007 are new and did not appear in the previous build (https://travis-ci.org/gap-system/gap-docker-pkg-tests-master/builds/331311839):

============================OUTPUT START==============================
#I  RunPackageTests("permut", "1.03", "tst/permut.tst", false);
 ┌───────┐   GAP 4.dev of today
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-pc-linux-gnu-default64
 Configuration:  gmp 5.1.3, readline
 Loaded workspace: wsp.g
 Packages:   FORMAT 1.3, GAPDoc 1.6.1, permut 1.03, PrimGrp 3.3.0, 
             smallgrp 1.2, TransGrp 2.0.2
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'
########> Diff in /home/gap/inst/gap-master/pkg/permut/tst/permut.tst:22
# Input is:
IsTGroup(SymmetricGroup(3));
# Expected output:
true
# But found:
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `RemoveSet' on 2 arguments
########
########> Diff in /home/gap/inst/gap-master/pkg/permut/tst/permut.tst:28
# Input is:
IdsOfAllSmallGroups(Size, 48, IsPTGroup, true) =
[ [ 48, 1 ], [ 48, 2 ], [ 48, 4 ], [ 48, 5 ], [ 48, 9 ], [ 48, 10 ], 
  [ 48, 11 ], [ 48, 20 ], [ 48, 23 ], [ 48, 24 ], [ 48, 34 ], [ 48, 35 ], 
  [ 48, 40 ], [ 48, 42 ], [ 48, 44 ], [ 48, 46 ], [ 48, 51 ], [ 48, 52 ] ];
# Expected output:
true
# But found:
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `RemoveSet' on 2 arguments
########
permut.tst
msecs: 116
#I  Errors detected while testing package permut 1.03
#I  using the test file `tst/permut.tst'
#I  RunPackageTests("permut", "1.03", "tst/permut.tst", false): runtime 328
============================OUTPUT END================================

[fingolfin]

This is caused by PR #2094, specifcally this code in grp.gi:

InstallMethod( IsPNilpotentOp,
    "for a group with special pcgs: test for normal Hall subgroup",
    [ IsGroup and HasSpecialPcgs, IsPosInt ],
    function( G, p )

    local primes, S;

    primes:= PrimeDivisors( Size( G ) );
    RemoveSet( primes, p );             # <--  this tries to remove an element from an immutable list
    S:= HallSubgroup( G, primes );

    return S <> fail and IsNormal( G, S );
    end );

Simple fix is to use Difference or Filtered instead of RemoveSet.

And I guess that teaches that it would have been best in PR #2094 to add tests that ensure every single changed line actually gets executed ... sigh