gigasecond: test prog rewrite

This addresses the first point of https://github.com/exercism/xgo/issues/110 by removing the requirement to define a Gigasecond value.  The second point of https://github.com/exercism/xgo/issues/110 doesn't seem called for by either the problem or the Go documentation of the time.Time type.  The Go documentation says "[Changing the location] changes only the presentation; it does not change the instant in time being denoted and therefore does not affect the computations...."  I see location as a non-issue for this problem.

For the exercise to "enter your birthday," the solver must now define a variable in the submitted program.  This allows discussion on various ways to initialize a time.Time.  Also TestVersion is added.  These are the API changes.

The test program now uses common test data from x-common.  Example_gen.go generates this.  Example_gen.go has a different structure than the one for clock.  This one uses Go text/templates as a step toward reuse of generation code in the future.