lib: split strings suffix/contains/index/compare tests per ref/hare/strings

Pure move: the hasprefix/hassuffix, contains, byteindex/rbyteindex/
index/rindex, and compare sections of strings_test.ww become sibling
suffix_test.ww, contains_test.ww, index_test.ww, compare_test.ww,
mirroring ref/hare/strings/{suffix,contains,index,compare}.ha
ownership. Blocks are byte-identical; only the section banner lines
are deleted (the file names now carry them).

The remaining families (dup, concat, trim, sub, utf8, iter, tokenize,
pad, replace, cut) stay in strings_test.ww: they all consume the
file-local streq helper, and a helper cannot be shared across split
test files — a single-file test root does not pull same-package
siblings (undefined: helper), while dir-mode composes all package
test files into one unit (duplicate fn helper). Splitting them would
force either helper duplication or renames, both out of scope for a
pure move.

Consumers: Makefile LIBRARY_TESTS gains the four new entries beside
the strings_test.ww row; test/byteid/libbyteid_test.ww roster gains
four fx rows, NENTEXPECT 44->48 (+4).

Makefile

@@ -486,6 +486,8 @@ LIBRARY_TESTS = lib/errors/errno_test.ww lib/ascii/ascii_test.ww \
 	  lib/ww/syntax/sym_test.ww \
 	  lib/strconv/test/ftos_test.ww lib/strconv/test/stof_test.ww \
 	  lib/strconv/test/int_test.ww lib/strings/strings_test.ww \
+	  lib/strings/suffix_test.ww lib/strings/contains_test.ww \
+	  lib/strings/index_test.ww lib/strings/compare_test.ww \
 	  lib/bytes/bytes_test.ww lib/encoding/utf8/utf8_test.ww \
 	  lib/bufio/bufio_test.ww lib/math/random/random_test.ww \
 	  lib/math/checked/checked_test.ww lib/fmt/fmt_test.ww \