Update logic for Parallel option with num_machines=1

[llxia]

When running tests with Parallel option and num_machines=1, run tests serially.
Don't do parallel as it will create unnecessary overhead

resolve: #4093
Signed-off-by: Lan Xia Lan_Xia@ca.ibm.com

[llxia]

• PARALLEL=Dynamic and NUM_MACHINES is empty: https://ci.adoptopenjdk.net/view/Test_grinder/job/Grinder/6077 ===> no parallel was launched as expected
• PARALLEL=Dynamic and NUM_MACHINES=1: https://ci.adoptopenjdk.net/view/Test_grinder/job/Grinder/6082 ===> no parallel was launched as expected
• PARALLEL=NodesByIterations and NUM_MACHINES=1: https://ci.adoptopenjdk.net/view/Test_grinder/job/Grinder/6081 ===> no parallel was launched as expected

[LongyuZhang]

LGTM

[smlambert]

thanks @llxia !

[AdamBrousseau]

Is NUM_MACHINES calculated or taken only from the param?
Thinking of the case where parallelx4 is requested but only 1 machine satisfies the label(s). Will we end up launching 1 child job or will this change catch that case?

[llxia]

Is NUM_MACHINES calculated or taken only from the param?
Thinking of the case where parallelx4 is requested but only 1 machine satisfies the label(s). Will we end up launching 1 child job or will this change catch that case?

We take NUM_MACHINES from the param and we will do check. It cannot be more than the number of available machines. In the above case, we end up launching 1 child job.