pp.c:do_chomp: Remove redundant assignment

The value of s is not used after this.
lex_assign.t: Don’t skip all failing tests

This was emasculated by 5cd1152e1.

Also, yank some crazy skip code that goes like this:

    if ($condition) {
       SKIP: {
          skip "reason", 1; # unconditionally
          pass(); # unreached
       }
    }
lex_assign.t: Add comment explaining this file
op_private: Update concat TARGLEX comment

This changed in 69b47968fa.
Don’t allow OPpTARGET_MY with integer negation

$ ./perl -Ilib -le 'use integer; my $a = "fake"; $a = -$a; print "[$a]"'
[--]

As of 1c2b3fd6f1, negation under ‘use integer’ can do string negation,
which modifies the return value before reading the argument.  So, like
regular non-integer negation, it must forego this optimisation.
Call STORE on lexical $tied = vec/chr

The assignment gets optimised away, so pp_vec and pp_chr need to call
SvSETMAGIC on their targets.

This bug started happening with vec just recently, in db098081, when
the OPpTARGET_MY optimisation was applied to it.

The bug started happening with chr for the same reason, but back in
5.6.0 (b162f9ea).

I moved the STORE tests down in lex_assign.t when expanding them, to
avoid sabotaging the tests that check the results of the expressions
after the __END__ marker.