bpf/tests: Reduce memory footprint of test suite

The test suite used to call any fill_helper callbacks to generate eBPF
program data for all test cases at once. This caused ballooning memory
requirements as more extensive test cases were added. Now the each
fill_helper is called before the test is run and the allocated memory
released afterwards, before the next test case is processed.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210914091842.4186267-3-johan.almbladh@anyfinetworks.com