tests: Call open() with the right flag

Build failed on the latest glibc (I think?), which caused this weird error:
/usr/include/bits/fcntl2.h:50:11: error: call to ‘__open_missing_mode’ declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments

In these three calls, open() was being called with 'r' flag, whose hex value is
0x72, and happens to set the O_CREAT flag (0x40) which was causing this error.
The correct flag to pass is O_RDONLY.

This issue exists since the creation of that file, I’m surprised it was working
previously.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>