+# __test_file, __apps_file and __fuzz_file return the full path to a file
+# relative to the test/, apps/ or fuzz/ directory in the build tree or the
+# source tree, depending on where the file is found. Note that when looking
+# in the build tree, the file name with an added extension is looked for, if
+# an extension is given. The intent is to look for executable binaries (in
+# the build tree) or possibly scripts (in the source tree).
+# These functions all take the same arguments as File::Spec::Functions::catfile,
+# *plus* a mandatory extension argument. This extension argument can be undef,
+# and is ignored in such a case.