Fix unit tests that fail because of culture specific date string comparisons

Fixes all date comparison errors. Only have a single unit test failure left to diagnose:

```
Kudu.Services.Test.ResumableLogReaderFacts.ResumableLogReaderCanHandleFileAccessError: System.ArgumentOutOfRangeException : The UTC time represented when the offset is applied must be between year 0 and 10,000.
Parameter name: offset
   at System.DateTimeOffset.ValidateDate(DateTime dateTime, TimeSpan offset)
   at System.DateTimeOffset..ctor(DateTime dateTime)
   at Kudu.Services.Diagnostics.ApplicationLogsReader.ResumableLogFileReader..ctor(FileInfoBase fileInfo, ITracer tracer, LogFileAccessStats stats) in c:\Dev\Projects\github\kudu\Kudu.Services\Diagnostics\ApplicationLogsReader.cs:line 215
   at Kudu.Services.Test.ResumableLogReaderFacts.ResumableLogReaderCanHandleFileAccessError() in c:\Dev\Projects\github\kudu\Kudu.Services.Test\ApplicationLogsReaderFacts.cs:line 399```