Mix.Tasks.Test.Coverage warns on failure (#11457)

Adds an explicit warning when exiting with an error code because of a
failed test coverage threshold.

Example:
```
-----------|--------------------------
    62.35% | Total

Coverage test failed, threshold not met:
        Coverage:   62.35%
        Threshold: 100.00%
```

Author: agundy

lib/mix/lib/mix/tasks/test.coverage.ex

@@ -299,6 +299,7 @@ defmodule Mix.Tasks.Test.Coverage do
     print_summary(module_results, totals, summary_opts)
 
     if totals < get_threshold(summary_opts) do
+      print_failed_threshold(totals, get_threshold(summary_opts))
       System.at_exit(fn _ -> exit({:shutdown, 3}) end)
     end
 
@@ -353,6 +354,13 @@ defmodule Mix.Tasks.Test.Coverage do
     Mix.shell().info("")
   end
 
+  defp print_failed_threshold(totals, threshold) do
+    Mix.shell().info("Coverage test failed, threshold not met:\n")
+    Mix.shell().info("    Coverage:  #{format_number(totals, 6)}%")
+    Mix.shell().info("    Threshold: #{format_number(threshold, 6)}%")
+    Mix.shell().info("")
+  end
+
   defp display({percentage, name}, threshold) do
     Mix.shell().info([
       color(percentage, threshold),
@@ -369,6 +377,9 @@ defmodule Mix.Tasks.Test.Coverage do
   defp color(percentage, threshold) when percentage >= threshold, do: :green
   defp color(_, _), do: :red
 
+  defp format_number(number, length) when is_integer(number),
+    do: format_number(number / 1, length)
+
   defp format_number(number, length), do: :io_lib.format("~#{length}.2f", [number])
 
   defp format_name(name) when is_binary(name), do: name

lib/mix/test/mix/tasks/test_test.exs

@@ -168,7 +168,14 @@ defmodule Mix.Tasks.TestTest do
 
         # We skip a test in bar to force coverage below the default threshold
         # which should result in an exit status of 1.
-        assert {_, code} = mix_code(["test", "--cover", "--exclude", "maybe_skip"])
+        assert {output, code} = mix_code(["test", "--cover", "--exclude", "maybe_skip"])
+
+        assert output =~ """
+               Coverage test failed, threshold not met:
+
+                   Coverage:    0.00%
+                   Threshold:  90.00%
+               """
 
         unless windows?() do
           assert code == 3