File I/O tests failing due to lack of disk space in CI on macOS

[stephentoub]

https://ci.dot.net/job/dotnet_coreclr/job/master/job/x64_checked_osx10.12_innerloop_prtest/7568/consoleText

Five I/O tests failed due to lack of space:

file_io.GetFileSize.test1
file_io.GetFileSizeEx.test1
file_io.SetFilePointer.test5
file_io.SetFilePointer.test6
file_io.SetFilePointer.test7

e.g.

........................................................................................................................GetFileSize: ERROR -> Call to SetEndOfFile failed with 112.


FAILED: file_io/GetFileSize/test1/paltest_getfilesize_test1. Exit code: 1

.GetFileSizeEx: ERROR -> SetEndOfFile failed due to lack of disk space


FAILED: file_io/GetFileSizeEx/test1/paltest_getfilesizeex_test1. Exit code: 1

............................................SetFilePointer: ERROR -> SetEndOfFile failed due to lack of disk space