Feature request: allow running tests without GAPDoc loaded

[fingolfin]

Right now I see this:

$ ./gap --bare
 ┌───────┐   GAP 4.15dev-174-g9608f02 built on 2025-08-14 22:35:58+0200
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: aarch64-apple-darwin21-default64-kv10
 Configuration:  gmp 6.3.0, GASMAN, readline
 Loading the library and packages ...
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'
gap> Test("tst/testinstall/rat
ratfun.tst      ratfun_gf5.tst  rationals.tst
gap> Test("tst/testinstall/rationals.tst");
Error, Variable: 'TextAttr' must have an assigned value in
  BindGlobal( "DefaultReportDiffColors", rec(
    message := TextAttr.4,
    input := "",
    expected := Concatenation( TextAttr.0, TextAttr.b2 ),
    actual := Concatenation( TextAttr.7, TextAttr.b1 ) ) ); at /Users/mhorn/Projekte/GAP/gap.spielwiese/lib/test.gi:655 called from
<function "Test">( <arguments> )
 called from read-eval loop at *stdin*:1
type 'quit;' to quit to outer loop
brk>

One simple fix would be to just not use TextAttr if it is not defined. The downside of doing this naively is that if later GAPDoc gets loaded, then we are stuck with no colors being used... which perhaps is good enough anyway. A more sophisticated implementation could take care of that, though; e.g. by turning DefaultReportDiffColors into a function, or using the package extension mechanism (to execute some code once GAPDoc is loaded, or or or...)

I (re)noticed this problem as part of doing gap-system/PackageDistro#1183 which in turn is motivated by issue #2434 (about GAP not requiring transgrp/primgrp/smallgrp)