entr: add test and modernize

Closes Homebrew/homebrew#35780.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/entr.rb

@@ -1,9 +1,7 @@
-require 'formula'
-
 class Entr < Formula
-  homepage 'http://entrproject.org/'
-  url 'http://entrproject.org/code/entr-3.0.tar.gz'
-  sha1 'e7c5f589b2bce839464052b116a051b4d8f43f23'
+  homepage "http://entrproject.org/"
+  url "http://entrproject.org/code/entr-3.0.tar.gz"
+  sha1 "e7c5f589b2bce839464052b116a051b4d8f43f23"
 
   bottle do
     cellar :any
@@ -13,10 +11,19 @@ class Entr < Formula
   end
 
   def install
-    ENV['PREFIX'] = prefix
-    ENV['MANPREFIX'] = man
+    ENV["PREFIX"] = prefix
+    ENV["MANPREFIX"] = man
     system "./configure"
     system "make"
-    system "make install"
+    system "make", "install"
+  end
+
+  test do
+    touch testpath/"test.1"
+    fork do
+      sleep 0.5
+      touch testpath/"test.2"
+    end
+    assert_equal "New File", pipe_output("#{bin}/entr -d echo 'New File'", testpath).strip
   end
 end