Fix DATADIR clash when build using MinGW64
- tests/Makefile.am: Change DATADIR to WGET_DATADIR
- unit-tests/Makefile.am: Likewise
- unit-tests/test.c: Likewise
When build Wget2 using MinGW64 and embedding other library (Libmicrohttpd), DATADIR macro from Wget2 clash with MinGW64 header.
This is follow up of #250 (closed)
Edited by username-removed-1278597