Implement STDIN isolation among tests (#43)

Isolate STDIN among tests and among a test and clitest itself.

This fixes the bug reported on issue #42 in which tests and clitest share STDIN,
causing unexpected results when the command specified on a test reads from
STDIN.

clitest

@@ -302,7 +302,7 @@ tt_run_test() {
     tt_debug EVAL "$tt_test_command"
 
     # Execute the test command, saving output (STDOUT and STDERR)
-    eval "$tt_test_command" > "$tt_test_output_file" 2>&1
+    eval "$tt_test_command" > "$tt_test_output_file" 2>&1 < /dev/null
     tt_test_exit_code=$?
 
     tt_debug OUTPUT "$(cat "$tt_test_output_file")"

test.md

@@ -2252,6 +2252,14 @@ OK: 10 of 10 tests passed
 $
 ```
 
+STDIN Isolation
+
+```
+$ ./clitest --quiet test/stdin-isolation.sh ; echo $?
+0
+$
+```
+
 Multiple commands in one line
 
 ```

test/stdin-isolation.sh

@@ -0,0 +1,12 @@
+#
+# This was a bug found on early versions of clitest in which tests shared
+# STDIN with clitest and with each other, causing unexpected results when
+# a test read from STDIN. This was reported on issue #42 on Github.
+#
+# Testing for a regression. 
+#
+
+$ echo testing stdin isolation ; read stdin_isolation
+testing stdin isolation
+$ echo Failed\? Regression to stdin isolation added. ; unset stdin_isolation
+Failed? Regression to stdin isolation added.