selftests/bpf: convert test_bpftool_metadata.sh into test_progs framework

selftests/bpf: convert test_bpftool_metadata.sh into test_progs framework

The test_bpftool_metadata.sh script validates that bpftool properly
returns in its ouptput any metadata generated by bpf programs through
some .rodata sections.

Port this test to the test_progs framework so that it can be executed
automatically in CI. The new test, similarly to the former script,
checks that valid data appears both for textual output and json output,
as well as for both data not used at all and used data. For the json
check part, the expected json string is hardcoded to avoid bringing a
new external dependency (eg: a json deserializer) for test_progs.
As the test is now converted into test_progs, remove the former script.

The newly converted test brings two new subtests:

  #37/1    bpftool_metadata/metadata_unused:OK
  #37/2    bpftool_metadata/metadata_used:OK
  #37      bpftool_metadata:OK
  Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20260123-bpftool-tests-v4-2-a6653a7f28e7@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>