encoding/asn1: fix time creation to also accommodate negative timezone offsets

[kimshrier]

When splitting the string representation of the time, the code did not previously allow for negative timezone offsets.

_{Huly®: V_0.6-21304}

[spytheman]

Can you please also add a test with a negative timezone?

[spytheman]

There is no need to restore the TZ env variable - it is local to the _test.v program, and will not affect other programs, after it completes.

[kimshrier]

I wasn't sure if the TZ environment variable setting would have an adverse effect on other tests in time_test.v.