Fix jacoco agent not added to command line (code coverage)

Fix jacoco agent not added to command line (code coverage)

It seems that tycho.testArgLine doesn't get set again by
tycho-surefire if it was set in the root properties.
I orginally defined it to nothing so that it would be defined
when running without jacoco. Instead, we can introduce a new
variable and never touch tycho.testArgLine.

Change-Id: I5f2969ad65292ad2e6fa84d3fb46bf925064981c
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/65953
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>