test: remove empty wpt/ directory (fully migrated to HTML cases)

All wpt tests are now in tests/cases/wpt_*/*.html (build.rs codegen).
The wpt/ directory only had empty mod.rs files (placeholders after
migration). Removed wpt/ + dropped 'mod wpt;' from tests/mod.rs.

Author: TtTRz

float-pigment-forest/tests/mod.rs

@@ -1,7 +1,5 @@
 #[cfg(test)]
 mod custom;
-#[cfg(test)]
-mod wpt;
 use euclid::num::Zero;
 pub use float_pigment_css::length_num::LengthNum;
 use float_pigment_css::{