t5300, t5302, t5303: Do not use /dev/zero

t5300, t5302, t5303: Do not use /dev/zero

We do not have /dev/zero on Windows. This replaces it by data generated
with printf, perl, or echo. Most of the cases do not depend on that the
data is a stream of zero bytes, so we use something printable; nor is an
unlimited stream of data needed, so we produce only as many bytes as the
test cases need.

Signed-off-by: Johannes Sixt <redacted>