diff-so-fancy: fix test

diff-so-fancy needs some actual diff as input, or it will just hang
there waiting for input from stdin.

Also, do not manually set TERM in test, which should no longer be
necessary since https://github.com/Homebrew/brew/pull/982.

Formula/diff-so-fancy.rb

@@ -22,7 +22,19 @@ class DiffSoFancy < Formula
   end
 
   test do
-    ENV["TERM"] = "xterm"
+    diff = <<-EOS.undent
-    system bin/"diff-so-fancy"
+      diff --git a/hello.c b/hello.c
+      index 8c15c31..0a9c78f 100644
+      --- a/hello.c
+      +++ b/hello.c
+      @@ -1,5 +1,5 @@
+       #include <stdio.h>
+
+       int main(int argc, char **argv) {
+      -    printf("Hello, world!\n");
+      +    printf("Hello, Homebrew!\n");
+       }
+    EOS
+    assert_match "modified: hello.c", pipe_output(bin/"diff-so-fancy", diff, 0)
   end
 end