tools build feature: Add some comments to explain the FEATURE_TESTS logic

The tools/build/feature/test-all.c works in conjunction with the
tools/build/Makefile.feature FEATURE_TESTS_BASIC and FEATURE_TESTS_EXTRA
contents, so that if test-all.c manages to be built, we go on and
iterate all entries in FEATURE_TESTS_BASIC + FEATURE_TESTS_EXTRA setting
them to 1.

To test this:

  $ rm -rf /tmp/b ; mkdir /tmp/b ; make -C tools/perf O=/tmp/b feature-dump
  $ cat /tmp/b/feature/test-all.make.output
  $ ldd /tmp/b/feature/test-all.bin
	linux-vdso.so.1 (0x00007f2a47a67000)
	libdw.so.1 => /lib64/libdw.so.1 (0x00007f2a477cf000)
	libpython3.12.so.1.0 => /lib64/libpython3.12.so.1.0 (0x00007f2a471fe000)
	libm.so.6 => /lib64/libm.so.6 (0x00007f2a4711a000)
	libtraceevent.so.1 => /lib64/libtraceevent.so.1 (0x00007f2a470f2000)
	libtracefs.so.1 => /lib64/libtracefs.so.1 (0x00007f2a470cb000)
	libcrypto.so.3 => /lib64/libcrypto.so.3 (0x00007f2a46c1b000)
	libz.so.1 => /lib64/libz.so.1 (0x00007f2a46bf8000)
	libbabeltrace-ctf.so.1 => /lib64/libbabeltrace-ctf.so.1 (0x00007f2a46bad000)
	libcapstone.so.5 => /lib64/libcapstone.so.5 (0x00007f2a464b8000)
	libopencsd_c_api.so.1 => /lib64/libopencsd_c_api.so.1 (0x00007f2a464a8000)
	libopencsd.so.1 => /lib64/libopencsd.so.1 (0x00007f2a46422000)
	libelf.so.1 => /lib64/libelf.so.1 (0x00007f2a46406000)
	libnuma.so.1 => /lib64/libnuma.so.1 (0x00007f2a463f6000)
	libslang.so.2 => /lib64/libslang.so.2 (0x00007f2a46113000)
	libperl.so.5.38 => /lib64/libperl.so.5.38 (0x00007f2a45d74000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f2a45b83000)
	liblzma.so.5 => /lib64/liblzma.so.5 (0x00007f2a45b50000)
	libzstd.so.1 => /lib64/libzstd.so.1 (0x00007f2a45a91000)
	libbz2.so.1 => /lib64/libbz2.so.1 (0x00007f2a45a7b000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f2a47a69000)
	libbabeltrace.so.1 => /lib64/libbabeltrace.so.1 (0x00007f2a45a6b000)
	libpopt.so.0 => /lib64/libpopt.so.0 (0x00007f2a45a5b000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x00007f2a45a51000)
	libgmodule-2.0.so.0 => /lib64/libgmodule-2.0.so.0 (0x00007f2a45a4a000)
	libglib-2.0.so.0 => /lib64/libglib-2.0.so.0 (0x00007f2a458fa000)
	libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007f2a45696000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f2a45668000)
	libcrypt.so.2 => /lib64/libcrypt.so.2 (0x00007f2a45630000)
	libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007f2a45590000)
  $ head /tmp/b/FEATURE-DUMP
  feature-backtrace=1
  feature-libdw=1
  feature-eventfd=1
  feature-fortify-source=1
  feature-get_current_dir_name=1
  feature-gettid=1
  feature-glibc=1
  feature-libbfd=1
  feature-libbfd-buildid=1
  feature-libcap=1
  $

There are inconsistencies that are being audited, as can be seen above
with the libcap case, that is not linked with test-all.bin nor is
present in test-all.c, so shouldn't be set as present. Further patches
are going to address those inconsistencies, but lets document this a bit
more to reduce the chances of this happening again.

Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/20241211224509.797827-2-acme@kernel.org
[ Fixed typo pointed out by Ian Rogers ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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