The units under the conformance test suite should be able to use
external files. Linking the files in tests/conform like the
interactive tests do seems like a hack piled on top of a hack, so
instead we should provide a programmatic way for a conformance
test unit to get the full path of a file, regardless of where the
tests/data directory is.
We can use a define to get the full path of tests/data and then
a function using g_build_filename() to construct the path to the
file we want.
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