lib/test: rename dotted-self-import tests to <mod>_test (#16 PREP-a2)

package L + import P.L is self-import in Go terms regardless of path
spelling (the parser drops the dotted prefix); the text-level
import-string!=package-name comparison hid these 3 from the PREP-a set.
All three verified black-box (qualifier-only exported-API use) — plain
rename, dotted imports kept as legit cross-package.

lib/encoding/utf8/utf8test.ww

@@ -3,7 +3,7 @@
 // then-fail()-with-+10 pattern as hex / base32 / time tests:
 // non-zero exit pinpoints the failing scenario.
 
-package utf8;
+package utf8_test;
 
 import bytes;
 import encoding.utf8;