Reduce duplication in reftests
Originally part of !3195 (closed).
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reftest-compare: Fix spelling of "diff"
This is the only non-whitespace difference between the copies in testsuite/reftests/ and testsuite/gsk/.
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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.