tests: fix "4 custom target depends extmodule" with Python 3.8+ on Windows

Since CPython 3.8 .pyd files no longer look in PATH for loading libraries,
but require the DLL directory to be explicitely added via os.add_dll_directory().
This resulted in those tests failing with 3.8+ on Windows.

Add the DLL build directory with os.add_dll_directory() to fix them.

This was never noticed in CI because it only uses Python 3.7 and the
MSYS2 CPython still used the old behaviour until now.

test cases/python/4 custom target depends extmodule/blaster.py

@@ -10,6 +10,9 @@ filedir = Path(os.path.dirname(__file__)).resolve()
 if list(filedir.glob('ext/*tachyon*')):
     sys.path.insert(0, (filedir / 'ext').as_posix())
 
+if hasattr(os, 'add_dll_directory'):
+    os.add_dll_directory(filedir / 'ext' / 'lib')
+
 import tachyon
 
 parser = argparse.ArgumentParser()

test cases/python3/4 custom target depends extmodule/blaster.py

@@ -10,6 +10,9 @@ filedir = Path(os.path.dirname(__file__)).resolve()
 if list(filedir.glob('ext/*tachyon.*')):
     sys.path.insert(0, (filedir / 'ext').as_posix())
 
+if hasattr(os, 'add_dll_directory'):
+    os.add_dll_directory(filedir / 'ext' / 'lib')
+
 import tachyon
 
 parser = argparse.ArgumentParser()