ucspi-tcp: use full path to executable, fix test failure

Closes Homebrew/homebrew#29477.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Formula/ucspi-tcp.rb

@@ -20,6 +20,8 @@ class UcspiTcp < Formula
   end
 
   test do
-    system "tcpserver"
+    out = `#{bin}/tcpserver 2>&1`
+    assert out.include?("usage: tcpserver")
+    assert_equal 100, $?.exitstatus
   end
 end