Move go version check into main() (#3734)

Previously the check was done in init().  This patch moves into main
for unit testable friendly function.

main_test.go

@@ -0,0 +1,50 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestCheckGoVersion(t *testing.T) {
+	// Test success cases.
+	testCases := []struct {
+		version     string
+		expectedErr error
+	}{
+		{minGoVersion, nil},
+		{minGoVersion + ".10", nil},
+		{"1.6.8", fmt.Errorf("Minio is not compiled by Go >= 1.7.  Please recompile accordingly.")},
+		{"0.1", fmt.Errorf("Minio is not compiled by Go >= 1.7.  Please recompile accordingly.")},
+		{".1", fmt.Errorf("Malformed version: .1")},
+		{"somejunk", fmt.Errorf("Malformed version: somejunk")},
+	}
+
+	for _, testCase := range testCases {
+		err := checkGoVersion(testCase.version)
+		if testCase.expectedErr == nil {
+			if err != nil {
+				t.Fatalf("expected: %v, got: %v", testCase.expectedErr, err)
+			}
+		} else if err == nil {
+			t.Fatalf("expected: %v, got: %v", testCase.expectedErr, err)
+		} else if testCase.expectedErr.Error() != err.Error() {
+			t.Fatalf("expected: %v, got: %v", testCase.expectedErr, err)
+		}
+	}
+}