Allow missing expect out and err files

It's annoying to have to create empty stdout and stderr files for
expect based unit tests. So make them optional.

.gitignore

@@ -7,7 +7,6 @@
 # which project directory they reside in.
 *.exe
 *.app
-*.out
 *.so
 *.dylib
 *.dll
@@ -35,5 +34,11 @@
 ._*
 Desktop.ini
 
+# This is a special suffix. There is exactly one directory where we expect
+# files with a `.out` extension: the ksh93 unit tests. So exclude it
+# everywhere then allow it in the ksh93 unit test directory.
+*.out
+!src/cmd/ksh93/tests/*.out
+
 # Artifacts of the build and test process that should not be committed.
 build/

src/cmd/ksh93/tests/util/run_test.sh

@@ -115,18 +115,34 @@ function run_interactive {
         >$test_name.out 2>$test_name.err
     exit_status=$?
 
-    if ! diff -q $TEST_SRC_DIR/$test_name.out $test_name.out >/dev/null
+    if [[ -e $TEST_SRC_DIR/$test_name.out ]]
     then
-        log_error "Stdout for $test_name had unexpected differences:"
-        diff -U3 $TEST_SRC_DIR/$test_name.out $test_name.out >&2
-        exit_status=1
+        if ! diff -q $TEST_SRC_DIR/$test_name.out $test_name.out >/dev/null
+        then
+            log_error "Stdout for $test_name had unexpected differences:"
+            diff -U3 $TEST_SRC_DIR/$test_name.out $test_name.out >&2
+            exit_status=1
+        fi
+    elif [[ -s $test_name.out ]]
+    then
+            log_error "Stdout for $test_name should have been empty:"
+            cat $test_name.out >&2
+            exit_status=1
     fi
 
-    if ! diff -q $TEST_SRC_DIR/$test_name.err $test_name.err >/dev/null
+    if [[ -e $TEST_SRC_DIR/$test_name.err ]]
+    then
+        if ! diff -q $TEST_SRC_DIR/$test_name.err $test_name.err >/dev/null
+        then
+            log_error "Stderr for $test_name had unexpected differences:"
+            diff -U3 $TEST_SRC_DIR/$test_name.err $test_name.err >&2
+            exit_status=1
+        fi
+    elif [[ -s $test_name.err ]]
     then
-        log_error "Stderr for $test_name had unexpected differences:"
-        diff -U3 $TEST_SRC_DIR/$test_name.err $test_name.err >&2
-        exit_status=1
+            log_error "Stderr for $test_name should have been empty:"
+            cat $test_name.err >&2
+            exit_status=1
     fi
 
     if [[ $exit_status -eq 0 ]]