Assuming that your directory structure looks like:
project
+---src
| test.txt
| proj.c
\---bin
a.out <- Working directory
your relative path is correct; your working directory is actually on the same level as the text file.
If you really mean that the file is up one directory as you stated, like this: (Note: This is an awkward project structure)
project
\---src
| test.txt
| proj.c
\---bin
a.out
or like this: (makes more sense)
project
| test.txt
+---src
| proj.c
\---bin
a.out
Then the path you need is "../test.txt"
or, equivalently, "../../project/test.txt"
A better location would be in a data directory, so your path would be "../data/test.txt"