Don't fail tests when the timestamp read resolution is higher than write.

Previously, if bup was able to read path timestamps at a higher
resolution than it could write them, tests would fail.  This situation
can occur (for example) when the stat() resolution is 1ns, but either
the underlying filesystem's isn't, or bup wasn't able to find
utimensat at build time.

To fix this, compute the maximum resolution of the test filesystem
(via "t/ns-timestamp-resolutions HERE"), and then limit the "bup
xstat" timestamp resolution in the tests to match via a new
--mtime-resolution argument.

For completeness, also add --atime-resolution and --ctime-resolution
arguments.

Thanks to Tim Riemenschneider <t.riemenschneider@detco.de> for
reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>