Avoid millions of rand() calls() when running tests

Avoid millions of rand() calls() when running tests

Many tests use a loop calling rand() to fill buffers with test data. As
these calls cannot be inlined, this adds up to noticeable overhead:
For example, running on QEMU RISC-V the test_write_format_7zip_large_copy
test took ~22 seconds before and with this change it's ~17 seconds.
This change uses a simpler xorshift64 random number generator that can be
inlined into the loop filling the data buffer. By default the seed for this
RNG is rand(), but it can be overwritten by setting the TEST_RANDOM_SEED
environment variable.

For a native build the difference is much less noticeable, but it's still
measurable: test_write_format_7zip_large_copy takes 314.9 ms ± 3.9 ms
before and 227.8 ms ± 5.8 ms after (i.e. 38% faster for that test).