selftests/bpf: Add compare_text_to_expected() helper

Some selftests generate text (such as BTF dump) and check it against
an expectation committed nearby.

There is no shared way to do that. prog_tests/btf_dump.c assembles an
"awk ... | diff -u" pipeline and hands it to system(). Any other test
wanting the same behaviour needs to reproduce both the comparison and
the reporting of a mismatch.

test_progs captures per-subtest output by pointing the stdout and
stderr FILE * globals at a memstream. But a child process inherits
descriptors, not the globals. So the "| diff -u" goes to the console
instead of the subtest log and is absent from the failure report.

Add a helper that compares two strings and runs diff(1) on mismatch,
properly relaying the output to stdout. Add tests for the helper.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260828215207.3105313-2-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>