test/pbkdf2: enlarge stdio rx buffer

[kfessel]

Contribution description

The pbkdf2-test contains transfers of upto 67 byte (65+2) 87 byte (64*8/6+1) (8/6 is BASE64) only limited by the speed of the serial connection.
The default rx buffer is 64 bytes -> if uart irqs are tight enough the bytes are not taken fast enough from the buffer and some are lost -> test fails

the failing test can be seen by sometimes failing on the emulated microbit in murdock.

The large writes are defined by pbkdf2/tests/02-random.py

This enlarges the buffer to 2^7 (128) bytes

Testing procedure

read and exec that test; sadly one might have to do multiple execs of that test to see it fail without this PR

Issues/PRs references

split out from #17918 as it is not that related to murdock

[maribu]

Murdock likes it and I really like not getting unrelated failures in the CI run anymore :)