Add collective test targets:

- (make check): GNU coding standards-compliant primary entry point,
  running all available tests in the best environment (i.e. Docker
  container).
- (make test-all-local): Local entry point, running only tests
  which do not require a special environment; intended for IDE
  integration and quick turnaround cycles.

Also modifies the Travis configuration to run (make check), to prevent
duplication.

.travis.yml

@@ -7,6 +7,4 @@ notifications:
   email: false
 
 script:
-  - make validate
-  - make test-unit
-  - make test-integration
+  - make check

Makefile

@@ -51,6 +51,8 @@ man:
 shell: build-container
 	$(DOCKER_RUN_DOCKER) bash
 
+check: validate test-unit test-integration
+
 test-integration: build-container
 	$(DOCKER_RUN_DOCKER) hack/make.sh test-integration
 
@@ -61,5 +63,11 @@ test-unit: build-container
 validate: build-container
 	$(DOCKER_RUN_DOCKER) hack/make.sh validate-git-marks validate-gofmt validate-lint validate-vet
 
+# This target is only intended for development, e.g. executing it from an IDE. Use (make test) for CI or pre-release testing. 
+test-all-local: validate-local test-unit-local
+
+validate-local:
+	hack/make.sh validate-git-marks validate-gofmt validate-lint validate-vet
+
 test-unit-local:
 	go test $$(go list -e ./... | grep -v '^github\.com/projectatomic/skopeo/\(integration\|vendor/.*\)$$')