selftests/bpf: Utility functions to capture libbpf log in test_progs

selftests/bpf: Utility functions to capture libbpf log in test_progs

Several test_progs tests already capture libbpf log in order to check
for some expected output, e.g bpf_tcp_ca.c, kfunc_dynptr_param.c,
log_buf.c and a few others.

This commit provides a, hopefully, simple API to capture libbpf log
w/o necessity to define new print callback in each test:

    /* Creates a global memstream capturing INFO and WARN level output
     * passed to libbpf_print_fn.
     * Returns 0 on success, negative value on failure.
     * On failure the description is printed using PRINT_FAIL and
     * current test case is marked as fail.
     */
    int start_libbpf_log_capture(void)

    /* Destroys global memstream created by start_libbpf_log_capture().
     * Returns a pointer to captured data which has to be freed.
     * Returned buffer is null terminated.
     */
    char *stop_libbpf_log_capture(void)

The intended usage is as follows:

    if (start_libbpf_log_capture())
            return;
    use_libbpf();
    char *log = stop_libbpf_log_capture();
    ASSERT_HAS_SUBSTR(log, "... expected ...", "expected some message");
    free(log);

As a safety measure, free(start_libbpf_log_capture()) is invoked in the
epilogue of the test_progs.c:run_one_test().

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240306104529.6453-6-eddyz87@gmail.com