Clone the required repo only in test build

[llxia]

The Openj9 repo is only needed for functional testing. For non-functional test builds (i.e., OpenJDK, system, etc), we do not need to clone the openj9 repo.

We can be more efficient by cloneing the required repo only. This will make test build run fast, save more machine time, and reduce unnecessary network activities.

[Adirala-Krishna]

@llxia Our VENDOR_TEST_REPO is not getting cloned after this commit and we are facing issues while running svt tests in jenkins.

Job link where tests ran before commit : Grinder_CR/8882/console
Rebuilding same job link without changing anything, Error: cannot find the following tests issue is seen here as repo is not getting cloned : Grinder_CR/8912/console

Error :
No tests found for target: testList
No parallel test lists generated.
Error: cannot find the following tests: mix.3hrs_1, mix.3hrs_2, mix.3hrs_3, mix.3hrs_4 (note: group target such as sanity is not accepted inside testList)
makeGen.mk:45: recipe for target 'autogen' failed

Test seems running fine when ran without this commit : Grinder_CR/8933/console

Can you please check on this. Thank you.

[llxia]

@Adirala-Krishna PR #4818 should address the issue

[Adirala-Krishna]

Issue got resolved, Thank you very much @llxia

[llxia]

PRs are delivered.