asset: add more test cases for ExternalKey.PubKey

[starius]

Added more derivation paths and corresponding public keys, check in the test that the values returned by the method match to expected values. Also added tpub (testnet xpub).

To run "chantools derivekey" with xpubs, use PR lightninglabs/chantools#179

[coveralls]

Pull Request Test Coverage Report for Build 12696712738

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 28 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.009%) to 40.825%

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Files with Coverage Reduction	New Missed Lines	%
tappsbt/create.go	2	53.22%
asset/asset.go	2	77.38%
fn/option.go	3	43.3%
tapchannel/aux_leaf_signer.go	3	43.43%
commitment/tap.go	3	83.64%
asset/mock.go	3	92.71%
tapchannel/aux_invoice_manager.go	6	83.25%
tapdb/multiverse.go	6	68.21%

Totals
Change from base Build 12689774591:	-0.009%
Covered Lines:	26535
Relevant Lines:	64997

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💛 - Coveralls

[ffranr]

Thank you very much for this! I've cherry picked your commit into #1284 so that we can avoid the merge commit. The commit still has you as the author and so on.