remove diag call in RT::Test::file_content

the main reason to remove it is it causes t/web/cf_access.t to fail no
warnings tests under TEST_VERBOSE=1 mode. the warning is like:

    Name "RT::Test::TODO" used only once: possible typo at .../Test/Builder.pm line 2244

though we can also get around it by increasing $Test::Builder::Level in
RT::Test::file_content or never call file_content in compilation stage(e.g. in
a BEGIN block or "use constant ..."), imo it doesn't worth it.
Merge branch '4.0/remove-test-file-content-diag' into 4.0-trunk