ci: run only custom_channels LiT itests

[ffranr]

Some lightning-terminal (LiT) itests are currently flaky. This commit restricts CI to running only the custom_channels subset of LiT itests.

[ffranr]

I haven't tested this locally. I'm waiting on CI to see if things run as expected.

[guggero]

Thanks for the fix! LGTM 🎉

[coveralls]

Pull Request Test Coverage Report for Build 10940243772

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 20 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.005%) to 40.42%

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Files with Coverage Reduction	New Missed Lines	%
tappsbt/create.go	2	53.22%
tapgarden/planter.go	2	74.87%
commitment/tap.go	6	83.91%
universe/interface.go	10	51.12%

Totals
Change from base Build 10924632032:	0.005%
Covered Lines:	24225
Relevant Lines:	59933

---

💛 - Coveralls

[gijswijs]

LGTM 🎉