Dieharder How can I make my input file suitable for Dieharder?
Personally I never bother with ASCII and just go binary as it's easier, but so be it. There's a lot of good stuff on this page, but in essence what you need to do is...
One.
To use it with an ascii formatted file:

     dieharder -g 202 -f your-file.txt -a

   (your-file.txt should consist of a header such as:

    #==================================================================
    # generator I did it myself
    #==================================================================
    type: d
    count: 100000
    numbit: 32
    3129711816
      85411969
    2545911541
    etc...

Two.
Please read https://crypto.stackexchange.com/a/61546/23115. This is important. Dieharder is a glutton and craves loads and loads of data. 2,500,000 ASCII characters is way too low and the result will be absolutely meaningless other than just failing.  The example in the linked answer sucked up 246 GB of binary bits so as not to rewind the data sample. That's more than a terabyte of ASCII.
Tip.
Randomness is a function of sample size. The size of your data set indicates alternative tests, such as the original diehard, or NIST's randomness test suite. The latter is pretty common in the literature for modern RNG designs, and I prefer it.

dieharder -a -g 201 -f your-file.bin to do it in binary without all that header nonsense.