Bump go version to 1.15.

Bumping go required bumping golangci-lint due to the following error:

  level=warning msg="[runner] Can't run linter goanalysis_metalinter: SA1023: failed prerequisites: [email protected]/gravitational/robotest/lib/loc"

Which is discussed here:

  golangci/golangci-lint#827

Bumping golangci-lint in turn required tweaking (or removing) a test
that would never run:

  lib/ssh/ssh_test.go:86:6: func `testEnv` is unused (unused)

I chose to enable the test, as ssh_test.go is not run regularly, and
I don't fully understand the impact of removing the env parameter from
RunAndParse. Better to let someone sort it out when they try to execute
ssh_test.go and have more context available.

Makefile

@@ -24,7 +24,7 @@ export DOCKER_TAG ?=
 export DOCKER_ARGS ?= --pull
 DOCKERFLAGS := --rm=true $(NOROOT) -v $(PWD):$(SRCDIR) -v $(BUILDDIR):$(SRCDIR)/build -w $(SRCDIR)
 BUILDBOX := robotest:buildbox
-GOLANGCI_LINT_VER ?= 1.21.0
+GOLANGCI_LINT_VER ?= 1.30.0
 
 .PHONY: help
 # kudos to https://gist.github.com/prwhite/8168133 for inspiration

docker/build/Dockerfile

@@ -11,7 +11,7 @@
 # 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.
-FROM quay.io/gravitational/debian-venti:go1.12.9-stretch
+FROM quay.io/gravitational/debian-venti:go1.15-buster
 
 ARG UID
 ARG GID

lib/ssh/ssh_test.go

@@ -50,7 +50,6 @@ func TestSshUtils(t *testing.T) {
 
 	t.Run("environment", func(t *testing.T) {
 		t.Parallel()
-		t.Skip() // this requires setup on sshd side, and we no longer use this method
 		testEnv(t, client)
 	})