chore: fixed timezone for tests

[milankonir]

The tests are failing when ran in a TZ different from GMT.

--- FAIL: TestEncodePartDefaultHeaders (0.00s)
    encode_test.go:71: Test output did not match testdata/encode/part-default-headers.golden
        To update golden file, run: go test -update
    encode_test.go:71: diff -want +got:
        | Content-Disposition: attachment; filename=stuff.zip; modification-date="01
        |- Feb 03 04:05 GMT"
        |+ Feb 03 05:05 CET"
        | Content-Id: <mycontentid>
        | Content-Transfer-Encoding: base64
        ...
        | WklQWklQWklQ
        | 

This PR fixates test run to TZ=Europe/London

[coveralls]

coverage: 86.283%. remained the same
when pulling df13d57 on milankonir:fix-timezone-for-tests
into 61b3eaf on jhillyerd:main.

[jhillyerd]

For the record, I am not in GMT/UTC, and am unable to duplicate the failure with any of:

env TZ=CET go test -race -count 1 ./...
env TZ=America/Los_Angeles go test -race -count 1 ./...
env TZ=Europe/London go test -race -count 1 ./...

Perhaps this is a change in a newer version of Go than the 1.20.11 I am running. Either way, I don't think altering the Makefile is the correct way to fix this, as IDEs and GitHub CI won't be using Makefiles.

Someone contributed a similar fix to one of my other projects in https://github.com/inbucket/inbucket/pull/406/files -- that's likely to be a better approach, if we can use it here.

[milankonir]

Interesting, figured out the test were failing for my TZ specifically. You can reproduce yourself by running:

TZ=Europe/Prague go test -race ./... 

I assume it has to do with zone abbreviation vs current locale behavior described in https://pkg.go.dev/time#Parse

Originally, I did not want to make code changes for the sake of tests, but formatting the mod date in UTC should not do any harm.

[jhillyerd]

Ah, yes, your example does fail for me, interesting.

Thanks for sending in the fix.