build.cmd should create a log file

[mikeharder]

I think build.cmd should create a log file, including things like detailed test results (including the time to execute each test). Would be nice to keep the 10 most recent logs (build.log, build.1.log ... build.9.log), but even a single log of the most recent build would be nice.

This came up when I ran build.cmd, and Kestrel unit tests took 120 seconds rather than the usual 6 seconds, but I cannot repro and I don't know what caused it (e.g. was it a single slow test or all tests). A build log showing unit test timing would have been helpful.

[muratg]

cc @pranavkm @anurse

[muratg]

@anurse Were you planning to add something like this in korebuild2?

[natemcmaster]

#159 started producing a file log to artifacts/msbuild/msbuild.log. Good enough?

[mikeharder]

LGTM