Add Filtering, HealthCheck, Better Tests (#350)

feat: add filtering when choosing merge requests (#346)
feat: Add healthcheck (#345)
refactor: Move to gomock (#349)
feat: Makes the remote of the plugin configurable (#348)

This is a #MINOR release.

cmd/mocks/helpers.go

@@ -0,0 +1,60 @@
+package mock_main
+
+import (
+	bytes "bytes"
+	io "io"
+	"net/http"
+	"testing"
+
+	gitlab "github.com/xanzy/go-gitlab"
+	gomock "go.uber.org/mock/gomock"
+)
+
+var MergeId = 3
+
+func NewListMrOptions() *gitlab.ListProjectMergeRequestsOptions {
+	return &gitlab.ListProjectMergeRequestsOptions{
+		Scope:        gitlab.Ptr("all"),
+		State:        gitlab.Ptr("opened"),
+		SourceBranch: gitlab.Ptr(""),
+	}
+}
+
+/* Make response makes a simple response value with the right status code */
+func makeResponse(status int) *gitlab.Response {
+	return &gitlab.Response{
+		Response: &http.Response{
+			StatusCode: status,
+		},
+	}
+}
+
+type MockOpts struct {
+	MergeId int
+}
+
+func NewMockClient(t *testing.T) *MockClientInterface {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+	mockObj := NewMockClientInterface(ctrl)
+	return mockObj
+}
+
+/** Adds a handler to satisfy the withMrs middleware by returning an MR from that endpoint with the given ID */
+func WithMr(t *testing.T, m *MockClientInterface) *MockClientInterface {
+	options := gitlab.ListProjectMergeRequestsOptions{
+		Scope:        gitlab.Ptr("all"),
+		State:        gitlab.Ptr("opened"),
+		SourceBranch: gitlab.Ptr(""),
+	}
+
+	m.EXPECT().ListProjectMergeRequests("", &options).Return([]*gitlab.MergeRequest{{IID: MergeId}}, makeResponse(http.StatusOK), nil)
+
+	return m
+}
+
+type MockAttachmentReader struct{}
+
+func (mf MockAttachmentReader) ReadFile(path string) (io.Reader, error) {
+	return bytes.NewReader([]byte{}), nil
+}