refactor: improve code structure and add tests

any.go

@@ -0,0 +1,26 @@
+package gomockctx
+
+import (
+	"context"
+
+	"github.com/golang/mock/gomock"
+)
+
+// Any returns a gomock.Matcher which matches any context.Context object.
+func Any() gomock.Matcher {
+	return &anyMatcher{}
+}
+
+type anyMatcher struct{}
+
+var _ gomock.Matcher = &anyMatcher{}
+
+func (cm *anyMatcher) Matches(x interface{}) bool {
+	_, ok := x.(context.Context)
+
+	return ok
+}
+
+func (cm *anyMatcher) String() string {
+	return "is a context.Context"
+}