Ensure that mix --cover displays correct coverage in an umbrella app (#8176)

`:cover.start/0` initializes the internal database where coverage data
on all compiled modules is stored. Prior to this change, the database
wasn't cleared out between sub-apps in an umbrella app. When
`:cover.analyse/2` is called, it runs against the entire contents of the
internal database. In an umbrella app, the coverage report for App1
would show App1, the report for App2 would show App1 and App2, App3
would have App1, App2, and App3, and so on.

Closes #8086

Author: gj

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

@@ -6,6 +6,7 @@ defmodule Mix.Tasks.Test do
 
     def start(compile_path, opts) do
       Mix.shell().info("Cover compiling modules ...")
+      _ = :cover.stop()
       _ = :cover.start()
 
       case :cover.compile_beam_directory(compile_path |> to_charlist) do

lib/mix/test/fixtures/umbrella_cover/apps/bar/lib/bar.ex

@@ -0,0 +1,5 @@
+defmodule Bar do
+  def hello do
+    :world
+  end
+end

lib/mix/test/fixtures/umbrella_cover/apps/bar/mix.exs

@@ -0,0 +1,13 @@
+defmodule Bar.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :bar,
+      version: "0.1.0",
+      # Choose something besides *_test.exs so that these test files don't
+      # get accidentally swept up into the actual Mix test suite.
+      test_pattern: "*_tests.exs"
+    ]
+  end
+end

lib/mix/test/fixtures/umbrella_cover/apps/bar/test/bar_tests.exs

@@ -0,0 +1,7 @@
+defmodule BarTest do
+  use ExUnit.Case
+
+  test "greets the world" do
+    assert Foo.hello() == :world
+  end
+end

lib/mix/test/fixtures/umbrella_cover/apps/bar/test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()

lib/mix/test/fixtures/umbrella_cover/apps/foo/lib/foo.ex

@@ -0,0 +1,5 @@
+defmodule Foo do
+  def hello do
+    :world
+  end
+end

lib/mix/test/fixtures/umbrella_cover/apps/foo/mix.exs

@@ -0,0 +1,13 @@
+defmodule Foo.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :foo,
+      version: "0.1.0",
+      # Choose something besides *_test.exs so that these test files don't
+      # get accidentally swept up into the actual Mix test suite.
+      test_pattern: "*_tests.exs"
+    ]
+  end
+end

lib/mix/test/fixtures/umbrella_cover/apps/foo/test/foo_tests.exs

@@ -0,0 +1,7 @@
+defmodule FooTest do
+  use ExUnit.Case
+
+  test "greets the world" do
+    assert Foo.hello() == :world
+  end
+end

lib/mix/test/fixtures/umbrella_cover/apps/foo/test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()

lib/mix/test/fixtures/umbrella_cover/mix.exs

@@ -0,0 +1,9 @@
+defmodule UmbrellaCover.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      apps_path: "apps"
+    ]
+  end
+end