syntax: add --lib to syntax tests

I couldn't figure how how to easily make doc tests run with 'no_std'
enabled, which regex-syntax now does. The '?' in particular was tripping
me up.

We still get doctest coverage from the top-level 'cargo test'.

Author: BurntSushi

regex-syntax/test

@@ -19,5 +19,8 @@ features=(
 )
 for f in "${features[@]}"; do
     echo "===== FEATURE: $f ==="
-    cargo test --no-default-features --features "$f"
+    # We only run library tests because I couldn't figure out how to easily
+    # make doc tests run in 'no_std' mode. In particular, without the Error
+    # trait, using '?' in doc tests seems tricky.
+    cargo test --no-default-features --lib --features "$f"
 done