Don't cast nanoseconds to integers when writing output

[khyperia]

I mostly care about catch_reporter_xml.cpp, but, figured I'd also do catch_reporter_console.cpp while I'm in the area.

Also, some context, since it's not plainly visible in the diff - stdev (just below) is already being written as doubles, without casting to int, so I don't think there's an issue with mean being written as a double as well

[codecov]

Codecov Report

Merging #1876 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1876   +/-   ##
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  Coverage   86.48%   86.48%           
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  Files         131      131           
  Lines        3899     3899           
=======================================
  Hits         3372     3372           
  Misses        527      527           

[horenmar]

I had some misgivings about the resulting formatting for ConsoleReporter (I don't want to tell users that something took 1.0000002213 ns), but it looks fine and there probably should be a more significant cleanup of how the times are written if I want it to be human readable.