Reduce duplication in reftests

Simon McVittie requested to merge wip/smcv/reftest-dedup into master

Originally part of !3195 (closed).

  • reftest-compare: Fix spelling of "diff"

    This is the only non-whitespace difference between the copies in testsuite/reftests/ and testsuite/gsk/.

  • reftests: Share reftest-compare.[ch] between GSK and GTK tests

    The only non-whitespace difference is that the copy in GSK does not set the symbol's visibility.

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