build unit tests with --coverage

wscript

@@ -42,4 +42,5 @@ def build(bld):
         includes = test_includes,
         defines = defines,
         lib = test_libs,
-        cxxflags = ["-Wall", "-std=gnu++14"])
+        cxxflags = ["-Wall", "-std=gnu++14", "--coverage"],
+        linkflags = ["--coverage"])