C++ migration: fix some Timestamp tests failing on Linux. (#999)

Some Timestamp tests check that constructor fails if given values beyond
the supported (pretty wide) range. In case Timestamp is created from
chrono::time_point, the time_point's underlying duration is
implementation-defined and may overflow when tests try to set it to
a value beyond Timestamp supported range. To work around this, make
these tests not run if time_point cannot represent boundary values on
this platform. For example, on OS X, time_point has microseconds
precision, so the tests may run, but on Linux, time_point has
nanoseconds precision, which would overflow.

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