use a generic Write instance for command output

cli/src/invocation/cmd/test.rs

@@ -1,3 +1,4 @@
+use std::io;
 use taskchampion::{server, taskstorage, Replica, ServerConfig};
 use tempdir::TempDir;
 
@@ -12,3 +13,38 @@ pub(super) fn test_server(dir: &TempDir) -> Box<dyn server::Server> {
     })
     .unwrap()
 }
+
+pub(super) struct TestWriter {
+    data: Vec<u8>,
+}
+
+impl TestWriter {
+    pub(super) fn into_string(self) -> String {
+        String::from_utf8(self.data).unwrap()
+    }
+}
+
+impl io::Write for TestWriter {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.data.write(buf)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.data.flush()
+    }
+}
+
+impl termcolor::WriteColor for TestWriter {
+    fn supports_color(&self) -> bool {
+        false
+    }
+    fn set_color(&mut self, _spec: &termcolor::ColorSpec) -> io::Result<()> {
+        Ok(())
+    }
+    fn reset(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+pub(super) fn test_writer() -> TestWriter {
+    TestWriter { data: vec![] }
+}