testsuite: Enlarge acceptance window for T1969
testsuite: Enlarge acceptance window for T1969

As noted in #17624, it's quite unstable, especially, for some reason, on
i386 and armv7 (something about 32-bit platforms perhaps?).

Metric Increase:
    T1969

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc