selftests: Fix selftests build to just build, not run tests

In commit 88baa78d ("selftests: remove duplicated all and clean
target"), the "all" target was removed from individual Makefiles and
added to lib.mk.

However the "all" target was added to lib.mk *after* the existing
"runtests" target. This means "runtests" becomes the first (default)
target for most of our Makefiles.

This has the effect of causing a plain "make" to build *and run* the
tests. Which is at best rude, but depending on which tests are run could
oops someone's build machine.

  $ make -C tools/testing/selftests/
  ...
  make[1]: Entering directory 'tools/testing/selftests/bpf'
  gcc -Wall -O2 -I../../../../usr/include   test_verifier.c -o tools/testing/selftests/bpf/test_verifier
  gcc -Wall -O2 -I../../../../usr/include   test_maps.c -o tools/testing/selftests/bpf/test_maps
  gcc -Wall -O2 -I../../../../usr/include   test_lru_map.c -o tools/testing/selftests/bpf/test_lru_map
  #0 add+sub+mul FAIL
  Failed to load prog 'Function not implemented'!
  #1 unreachable FAIL
  Unexpected error message!
  #2 unreachable2 FAIL
  ...

Fix it by moving the "all" target to the start of lib.mk, making it the
default target.

Fixes: 88baa78d ("selftests: remove duplicated all and clean target")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>