[spec/ysh-comand-sub] Failing test for @() and J8 lines

This is #1126.

Minor re-org in data_lang/

data_lang/README.md

@@ -18,20 +18,21 @@ Shared test code:
 
 C library used by CPython, using `malloc() realloc()`:
 
-    j8c.{h,c}  j8c_test.c
+    j8_libc.{h,c}  j8_libc_test.c
 
 Parser and printer in Python, translated by mycpp:
 
     j8.py
-
-
-## Packle
-
-TODO
+    pyj8.py     # function that can be optimized in C++
+    j8_lite.py  # string-only functions, used by ASDL runtime
+                # so it must have few dependencies
 
 ## UTF-8
 
 Tests for different implementations:
 
     utf8_test.cc
 
+## Packle
+
+TODO

data_lang/demo.sh → data_lang/quoting-survey.sh

@@ -6,7 +6,7 @@
 # - Those with terminal escape sequences
 #
 # Usage:
-#   data_lang/demo.sh <function name>
+#   data_lang/quoting-survey.sh <function name>
 
 set -o nounset
 set -o pipefail

spec/ysh-command-sub.test.sh

@@ -1,6 +1,8 @@
-# @( conflict
+## oils_failures_allowed: 1
+
+
+#### conflict with extglob @( cna be avoided with ,(
 
-#### extglob
 shopt -s extglob
 
 [[ foo.py == @(*.sh|*.py) ]]
@@ -63,7 +65,23 @@ b
 c
 ## END
 
-#### Idiomatic for loop using @(seq $n)
+#### @() decodes J8 Lines
+
+var b = @(
+  echo "  unquoted ";
+  # I guess this is allowed
+  echo $'binary \xff';
+  echo '"json\n\u03bc"';
+  echo "u'j8 u \\u{3bc}'";
+  echo "b'j8 b \\y{ff'";
+)
+
+pp line (b)
+
+## STDOUT:
+## END
+
+#### for loop using @(seq $n)
 shopt -s oil:upgrade
 
 for x in @(seq 3) {