selftests/bpf: Compare btf_dump expected output in-process

test_btf_dump_case() writes the dumped types to a temporary file and
then runs a command against the expectation:

	system("awk '...' 'progs/x.c' | diff -u - '/tmp/x.output.XXXXXX'");

This creates three processes per test case: a shell, awk and diff. The
awk cannot be deferred to the failure path, because its output is the
expected text.

In a VM with a 9p root (used on BPF CI) process creation dominates the
cost. A bare fork+exec measures around 110ms, so the btf_dump test
cases spend most of their runtime forking.

Replace the awk pipeline with a simple marker parser. Buffer the dump
in memory, and use compare_text_to_expected(), which only runs diff(1)
on mismatch.

Measured with "time ./test_progs -t btf_dump" in the VM:

	real 2.751s -> 0.702s, sys 1.901s -> 0.193s

This also drops the temporary file setup.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260828215207.3105313-4-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>