test codec packages only when installed

Author: bicycle1885

test/REQUIRE

@@ -1,2 +0,0 @@
-CodecZlib
-CodecZstd

test/runtests.jl

@@ -86,6 +86,8 @@ using Base.Test
     TranscodingStreams.test_roundtrip_transcode(Identity, Identity)
 end
 
-Pkg.test("CodecZlib")
-Pkg.test("CodecZstd")
-Pkg.test("CodecBzip2")
+for pkg in ["CodecZlib", "CodecZstd", "CodecBzip2"]
+    if Pkg.installed(pkg) !== nothing
+        Pkg.test(pkg)
+    end
+end