test-fileio: fix bogus error when /proc/cmdline contains newlines

test-fileio: fix bogus error when /proc/cmdline contains newlines

The kernel does not sanitize /proc/cmdline. E.g. when running under qemu, it is
easy to pass a string with newline by mistake. We use read_one_line_file(), so
we would read only the first list of the file, and
write_string_file(WRITE_STRING_FILE_VERIFY_ON_FAILURE) would fail because the
target file is obviously different. Change to a kernel-generated file to avoid
the issue.

v2:
- use /proc/version instead of /proc/uptime for attempted writes, so the test
  test passes even if test_write_string_file_verify() takes more than 10 ms ;]