Travis - Remove the t.parallel() from TestChannelLinkBidirectionalOneHopePayments.

[offerm]

This test is CPU intensive and creates ~966 goroutings. While the test itself is OK it has impact on other tests that are running in parallel. In these other tests we can find sleep(time) that is used for sync. Due to this test the sleep(time) may be too short and may cause these tests to fail.

[Roasbeef]

It shouldn't be an issue that this test is running, instead the other tests in the package may need to have their timing assumptions checked to ensure that they can execute in parallel with other tests that use a high number of goutines.

[offerm]

@Roasbeef Agree in general.

Still, since the impact of this test is random and the developer may see any of the other tests failing without a good reason, I suggest removing the parallel attribute from this test until

"the other tests in the package may need to have their timing assumptions checked to ensure that they can execute in parallel with other tests that use a high number of goutines."

Keep in mind that external contributors can't "restart" the Travis process upon such random failure and are basically blocked not knowing if what they did is actually good and not knowing if the PR (that failed the test) will be reviewed by the team.

Consider this as a temporary fix aiming to enhance productivity for the community.