ash: add tests for failures of the exec and command builtins

The exec builtin should return an exit status of 127 if the command
can't be found.  It doesn't:  it returns 2.

If the command builtin is used to source a script that runs a second
script that doesn't exist ash should issue an error.  Instead it seg
faults.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash_test/ash-misc/exec.tests b/shell/ash_test/ash-misc/exec.tests
new file mode 100755
index 0000000..624915d
--- /dev/null
+++ b/shell/ash_test/ash-misc/exec.tests
@@ -0,0 +1,3 @@
+rm -f test1.sh
+(exec ./test1.sh)
+echo $?