[BUG] NEIGHBORS_ALL_NEIGHBORS_TEST build ignores --no-mg

[enp1s0]

The following error occurs when building the unit tests on a machine where NCCL is not installed, even though the --no-mg option is specified.

CMake Error at tests/CMakeLists.txt:47 (target_link_libraries):
  Target "NEIGHBORS_ALL_NEIGHBORS_TEST" links to:

    NCCL::NCCL

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  tests/CMakeLists.txt:213 (ConfigureTest)

This is because NCCL::NCCL is specified in tests/CmakeLists.txt

cuvs/cpp/tests/CMakeLists.txt

Lines 213 to 216 in afc24ee

	ConfigureTest(
	NAME NEIGHBORS_ALL_NEIGHBORS_TEST PATH neighbors/all_neighbors/test_float.cu GPUS 1 PERCENT 100
	ADDITIONAL_DEP NCCL::NCCL
	)

I could build the unit tests without the NCCL dependency. Is this dependency really necessary even when the --no-mg option is specified?
This PR modifies the CMakeLists.txt file so that the NCCL dependency is set only when the --no-mg option is not specified.

Rel: #785 vis: @jinsolp

[robertmaynard]

This can be simplified to:

ConfigureTest(
    NAME NEIGHBORS_ALL_NEIGHBORS_TEST PATH neighbors/all_neighbors/test_float.cu GPUS 1 PERCENT 100
    ADDITIONAL_DEP $<TARGET_NAME_IF_EXISTS:NCCL::NCCL>
  )

[jinsolp]

Thanks for finding this! We can just get rid of this NCCL line altogether because this doesn't depend on nccl at all.

[enp1s0]

Thank you @jinsolp for letting me know. I removed the line.

[cjnolet]

@robertmaynard mind re-reviewing when you have a moment?

[cjnolet]

/merge