Use DIRECTORY_SEPARATOR instead of / in test. Fixes codeguy/Slim/#896.

ViewTest::testSetTemplatesDirectory() has hardcoded "/" character which
causes tests to fail in Windows. Code itself is correct. Only test is
not portable.

Patch has not been tested with actual Windows machine.
Merge pull request #897 from tuupola/896-windows-directory-separator

Use DIRECTORY_SEPARATOR instead of / in test.