[bitcoin/bitcoin] appveyor: Run functional test on appveyor (#14007)

MarcoFalke commented on this pull request.

> @@ -77,7 +77,7 @@ def bctest(testDir, testObj, buildenv):
are not as expected. Error is caught by bctester() and reported.
«»»
# Get the exec names and arguments
— execprog = os.path.join(buildenv[«BUILDDIR»], «src», testObj[«exec»] + buildenv[«EXEEXT»])
+ execprog = os.path.join(os.environ.get(‘BITCOINEXEPATH’, ») or os.path.join(buildenv[«BUILDDIR»], «src»), testObj[«exec»] + buildenv[«EXEEXT»])

Why is this change needed?