utf8: add tests for behavior change in v5.15.6-407-gc710240, and more

In v5.15.6-407-gc710240 Father Chrysostomos patched utf8::decode() so it
would call SvPV_force_nolen() on its argument. This meant that calling
utf8::decode() with a non-blessed non-overloaded reference would now
coerce the reference scalar to a string, i.e. before we'd do:

    $ ./perl -Ilib -MDevel::Peek -wle 'use strict; print $]; my $s = shift; my $s_ref = \$s; utf8::decode($s_ref); Dump $s_ref; print $$s_ref' ævar
    5.019011
    SV = IV(0x2579fd8) at 0x2579fe8
      REFCNT = 1
      FLAGS = (PADMY,ROK)
      RV = 0x25c33d8
      SV = PV(0x257ab08) at 0x25c33d8
        REFCNT = 2
        FLAGS = (PADMY,POK,pPOK)
        PV = 0x25a1338 "\303\246var"\0
        CUR = 5
        LEN = 16
    ævar

But after calling SvPV_force_nolen(sv) we'd instead do:

    $ ./perl -Ilib -MDevel::Peek -wle 'use strict; print $]; my $s = shift; my $s_ref = \$s; utf8::decode($s_ref); Dump $s_ref; print $$s_ref' ævar
    5.019011
    SV = PVIV(0x140e4b8) at 0x13e7fe8
      REFCNT = 1
      FLAGS = (PADMY,POK,pPOK)
      IV = 0
      PV = 0x140c578 "SCALAR(0x14313d8)"\0
      CUR = 17
      LEN = 24
    Can't use string ("SCALAR(0x14313d8)") as a SCALAR ref while "strict refs" in use at -e line 1.

I think this is arguably the right thing to do, we wouln't actually utf8
decode the containing scalar so this reveals bugs in code that passed
references to utf8::decode(), what you want is to do this instead:

    $ ./perl -CO -Ilib -MDevel::Peek -wle 'use strict; print $]; my $s = shift; my $s_ref = \$s; utf8::decode($$s_ref); Dump $s_ref; print $$s_ref' ævar
    5.019011
    SV = IV(0x1aa8fd8) at 0x1aa8fe8
      REFCNT = 1
      FLAGS = (PADMY,ROK)
      RV = 0x1af23d8
      SV = PV(0x1aa9b08) at 0x1af23d8
        REFCNT = 2
        FLAGS = (PADMY,POK,pPOK,UTF8)
        PV = 0x1ad0338 "\303\246var"\0 [UTF8 "\x{e6}var"]
        CUR = 5
        LEN = 16
    ævar

However I think we should be more consistent here, e.g. we'll die when
utf8::upgrade() gets passed a reference, but utf8::downgrade() just
passes it through. I'll file a bug for that separately.