unittest: default no color if stdout is not a tty

We accept a new environment variable, NIMTEST_COLOR,
which override the effect of NIMTEST_NO_COLOR.
The environment variable, NIMTEST_COLOR, can be 'never'
or 'always', which set the color output to false or true,
respectively.

lib/pure/unittest.nim

@@ -121,9 +121,16 @@ type
   ConsoleOutputFormatter* = ref object of OutputFormatter
     colorOutput: bool
       ## Have test results printed in color.
-      ## Default is true for the non-js target
-      ## unless, the environment variable
-      ## ``NIMTEST_NO_COLOR`` is set.
+      ## Default is true for the non-js target,
+      ## for which ``stdout`` is a tty.
+      ## Setting the environment variable
+      ## ``NIMTEST_COLOR`` to ``always`` or
+      ## ``never`` changes the default for the
+      ## non-js target to true or false respectively.
+      ## The deprecated environment variable
+      ## ``NIMTEST_NO_COLOR``, when set,
+      ## changes the defualt to true, if
+      ## ``NIMTEST_COLOR`` is undefined.
     outputLevel: OutputLevel
       ## Set the verbosity of test results.
       ## Default is ``PRINT_ALL``, unless
@@ -185,7 +192,15 @@ proc defaultConsoleFormatter*(): ConsoleOutputFormatter =
     # Reading settings
     # On a terminal this branch is executed
     var envOutLvl = os.getEnv("NIMTEST_OUTPUT_LVL").string
-    var colorOutput  = not existsEnv("NIMTEST_NO_COLOR")
+    var colorOutput  = isatty(stdout)
+    if existsEnv("NIMTEST_COLOR"):
+      let colorEnv = getenv("NIMTEST_COLOR")
+      if colorEnv == "never":
+        colorOutput = false
+      elif colorEnv == "always":
+        colorOutput = true
+    elif existsEnv("NIMTEST_NO_COLOR"):
+      colorOutput = false
     var outputLevel = PRINT_ALL
     if envOutLvl.len > 0:
       for opt in countup(low(OutputLevel), high(OutputLevel)):

tests/testament/tester.nim

@@ -427,7 +427,7 @@ include categories
 #                 if status: reSuccess else: reOutputsDiffer)
 
 proc main() =
-  os.putenv "NIMTEST_NO_COLOR", "1"
+  os.putenv "NIMTEST_COLOR", "never"
   os.putenv "NIMTEST_OUTPUT_LVL", "PRINT_FAILURES"
 
   backend.open()