travis.yml: disable unit tests on travis

[mvo5]

We already run our unit tests as a github action and also run them
as part of the spread "tests/unit/go/" testing. So this commit
removes them from running on travis.

[zyga]

There's a "go master/xenial static and unit test suite" below, does that also need to go?

[mvo5]

@zyga I kept it for now because it's nice to run out tests against go master as an early indication if anything breaks. We could move this to GH actions too via the "go" snap and the latest channel (?).

[zyga]

Thanks, I missed the fact that "master" is "go master" not "snapd master".

[zyga]

@mvo5 this failed on ...

----------------------------------------------------------------------

FAIL: cmd_sign_build_test.go:73: SnapSignBuildSuite.TestSignBuildWorks

cmd_sign_build_test.go:91:

    c.Assert(err, IsNil)

... value *errors.errorString = &errors.errorString{s:"cannot sign assertion: cannot sign assertion: bad GPG produced signature: it does not verify: openpgp: invalid signature: RSA verification failure"} ("cannot sign assertion: cannot sign assertion: bad GPG produced signature: it does not verify: openpgp: invalid signature: RSA verification failure")