regress: avoid backporting a cmd subst bug from beta

ksh 93v- beta introduced a regression with nested command
substitutions: backticks nested in $( ) result in misdirected
output. This has never been in 93u+, but since we're often
backporting things, let's avoid backporting this bug. It is also
useful if this shows up when running our bin/shtests against the
actual beta by adding a SHELL=... argument.
Ref.: https://github.com/att/ast/issues/478

src/cmd/ksh93/tests/subshell.sh:
- Add reproducer submitted by the reporter as a regression test.

src/cmd/ksh93/tests/subshell.sh

@@ -336,6 +336,17 @@ do	for ((TEST=1; TEST<=${#testcase[@]}; TEST++))
 	shift 2
 done
 
+# Regression introduced in 93v- beta; let's make sure not to backport it to 93u+m
+# Ref.: https://github.com/att/ast/issues/478
+expected='foo=bar'
+actual=$(
+	foo=$(print `/bin/echo bar`)   # should print nothing
+	print foo=$foo                 # should print "foo=bar"
+)
+[[ $actual == "$expected" ]] \
+|| err_exit 'Backticks nested in $( ) result in misdirected output' \
+	"(expected $(printf %q "$expect"), got $(printf $q "$actual"))"
+
 # the next tests loop on all combinations of
 #	{ SUB CAT INS TST APP } X { file-sizes }
 # where the file size starts at 1Ki and doubles up to and including 1Mi