test: add substitution tests.

check.sh

@@ -72,6 +72,8 @@ tests='
 30-substitution-backtick
 30-substitution-assignment
 30-for-substitution
+30-substitution-assignment
+30-substitution-redirect
 
 40-eval
 40-eval-echo-variable

test/30-substitution-assignment.sh

@@ -1,3 +1,2 @@
-f=test/test.sh
-b=test/$(basename $f .sh)
-echo b=$b
+echo=$(echo 1 2 3)
+echo echo=$echo

test/30-substitution-assignment.stdout

@@ -1 +1 @@
-b=test/test
+echo=1 2 3

test/30-substitution-redirect.sh

@@ -0,0 +1 @@
+echo $(echo foo bar baz 2>/dev/null)

test/30-substitution-redirect.stdout

@@ -0,0 +1 @@
+foo bar baz