Add test case for unit test and delete the duplicated docker file.

[weicongw]

Issue #, if available:

Description of changes:
Follow-up on the previous PR: #456. Add a test case for the unit test and remove the duplicated Dockerfile. The Nvidia GPU tests will use the same image for all tests (both single-node and multi-node).

Testing:

go test -v . -args -nvidiaTestImage  public.ecr.aws/o5d5x8n6/weicongw:nvidia -feature unit-test -efaEnabled=true
W0710 18:30:20.557612   14459 warnings.go:70] spec.template.spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution.nodeSelectorTerms[0].matchExpressions[0].key: beta.kubernetes.io/instance-type is deprecated since v1.17; use "node.kubernetes.io/instance-type" instead
W0710 18:30:20.557667   14459 warnings.go:70] spec.template.metadata.annotations[scheduler.alpha.kubernetes.io/critical-pod]: non-functional in v1.16+; use the "priorityClassName" field instead
2024/07/10 18:30:25 No node type specified. Using the node type p3.2xlarge in the node groups.
=== RUN   TestMPIJobPytorchTraining
=== RUN   TestMPIJobPytorchTraining/single-node
    env.go:438: Skipping feature "single-node": name not matched
=== RUN   TestMPIJobPytorchTraining/multi-node
    env.go:438: Skipping feature "multi-node": name not matched
--- PASS: TestMPIJobPytorchTraining (0.00s)
    --- SKIP: TestMPIJobPytorchTraining/single-node (0.00s)
    --- SKIP: TestMPIJobPytorchTraining/multi-node (0.00s)
=== RUN   TestSingleNodeUnitTest
=== RUN   TestSingleNodeUnitTest/unit-test
=== RUN   TestSingleNodeUnitTest/unit-test/Unit_test_Job_succeeds
--- PASS: TestSingleNodeUnitTest (25.48s)
    --- PASS: TestSingleNodeUnitTest/unit-test (25.48s)
        --- PASS: TestSingleNodeUnitTest/unit-test/Unit_test_Job_succeeds (25.07s)
PASS
ok      github.com/aws/aws-k8s-tester/e2e2/test/cases/nvidia    45.036s

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ndbaker1]

lgtm, one doc update