tests: write progress message to stderr, not stdout

We can argue that this message isn't really the "output" of the tests,
but is just a temporary communication to the user that the program
hasn't hung unexpectedly.

`unittest` writes everything to stdout, and so with this commit, the
user who compiles the tests and runs
```
$ ./tests/all_tests > /tmp/configlet_test_results.txt
```
will no longer see the message in that file - they see it in their
terminal instead.

tests/test_binary.nim

@@ -271,13 +271,12 @@ proc main =
   const helpStart = &"Usage:\n  {binaryName} [global-options] <command> [command-options]"
 
   const cmd = "nimble --verbose build -d:release"
-  stdout.write(&"Running `{cmd}`... ")
-  stdout.flushFile()
+  stderr.write(&"Running `{cmd}`... ")
   let (buildOutput, buildExitCode) = execCmdEx(cmd, workingDir = repoRootDir)
   if buildExitCode == 0:
-    echo "success"
+    stderr.writeLine "success"
   else:
-    echo "failure"
+    stderr.writeLine "failure"
     raise newException(OSError, buildOutput)
 
   suite "help as an argument":