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/crypto/sha256/sha256_test.ww

@@ -8,7 +8,7 @@
 // the rest of the 9xx stdlib tests; the non-zero exit pinpoints the
 // failing case. Expected digests come through the (separately tested)
 // hex.decodestr so the vectors stay readable.
-package sha256;
+package sha256_test;
 
 import bytes;
 import crypto.sha256;

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;

lib/math/checked/checked_test.ww

@@ -1,4 +1,4 @@
-package checked;
+package checked_test;
 
 // Directory import pulls both checked.ww and saturating.ww (the two
 // files of this module); a bare `import checked` would resolve only to