Disable ENABLE_TESTCOVERAGE on CentOS 7 build

[leezu]

Causes significant slowdown in connection with the system (devtoolset) provided OpenMP libraries.
#17559 starts pulling in the system OpenMP in addition to the 3rdparty/openmp causing the slowdown to materialize after the merge of #17559.

Fixes #19502

Let's still fix the issue of intgemm pulling a second openmp @kpuatamazon

With this PR

[2020-11-10T00:14:44.881Z] ============================= slowest 50 durations =============================

[2020-11-10T00:14:44.881Z] 467.92s call     tests/python/unittest/test_random.py::test_random

[2020-11-10T00:14:44.881Z] 74.58s call     tests/python/unittest/test_operator.py::test_image_normalize

[2020-11-10T00:14:44.881Z] 74.46s call     tests/python/unittest/test_random.py::test_randint_generator

[2020-11-10T00:14:44.881Z] 53.77s call     tests/python/unittest/test_random.py::test_shuffle

[2020-11-10T00:14:44.881Z] 29.70s call     tests/python/unittest/test_gluon.py::test_slice_pooling2d_slice_pooling2d

[2020-11-10T00:14:44.881Z] 19.73s call     tests/python/unittest/test_random.py::test_dirichlet

[2020-11-10T00:14:44.881Z] 15.36s call     tests/python/unittest/test_sparse_operator.py::test_sparse_mathematical_core

[2020-11-10T00:14:44.881Z] 10.90s call     tests/python/unittest/test_gluon.py::test_slice_pooling2d

[2020-11-10T00:14:44.881Z] 10.63s call     tests/python/unittest/test_operator.py::test_big_transpose

[2020-11-10T00:14:44.881Z] 10.30s call     tests/python/unittest/test_sparse_operator.py::test_sparse_square_sum

Without this PR

[2020-11-09T22:46:33.281Z] ============================= slowest 50 durations =============================

[2020-11-09T22:46:33.281Z] 599.70s call     tests/python/unittest/test_gluon.py::test_slice_pooling2d_slice_pooling2d

[2020-11-09T22:46:33.281Z] 246.20s call     tests/python/unittest/test_random.py::test_randint_generator

[2020-11-09T22:46:33.281Z] 219.33s call     tests/python/unittest/test_gluon.py::test_slice_pooling2d

[2020-11-09T22:46:33.281Z] 182.46s call     tests/python/unittest/test_random.py::test_random

[2020-11-09T22:46:33.281Z] 143.63s call     tests/python/unittest/test_sparse_operator.py::test_cast_storage_ex

[2020-11-09T22:46:33.281Z] 125.78s call     tests/python/unittest/test_sparse_operator.py::test_sparse_square_sum

[2020-11-09T22:46:33.281Z] 88.98s call     tests/python/unittest/test_random.py::test_negative_binomial_generator

[2020-11-09T22:46:33.281Z] 69.17s call     tests/python/unittest/test_random.py::test_poisson_generator

[2020-11-09T22:46:33.281Z] 63.23s call     tests/python/unittest/test_gluon.py::test_slice_batchnorm

[2020-11-09T22:46:33.281Z] 62.36s call     tests/python/unittest/test_gluon.py::test_slice_batchnorm_reshape_batchnorm

[mxnet-bot]

Hey @leezu , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

---

CI supported jobs: [sanity, windows-cpu, edge, centos-gpu, unix-cpu, unix-gpu, centos-cpu, windows-gpu, clang, website, miscellaneous]

---

Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[mseth10]

Verified that centos-cpu duration reduces to ~50mins from ~100mins after this commit. Thanks for the fix @leezu

[leezu]

@mseth10 any idea about the difference in tests/python/unittest/test_random.py::test_random?

[mseth10]

Seems to be an outlier, all other tests run faster. Should we re-trigger centos-cpu for a second datapoint?

[leezu]

I think we can merge the PR and observe the performance on the master branch.