libe2p: apply LDFLAGS when building tests

libe2p: apply LDFLAGS when building tests

This was previously addressed by Michael Forney on commit 53904ae543591a
but it seems the test cases from lib/e2p (tst_ostype and tst_feature)
were missed.

Fix them by adding the same $(ALL_LDFLAGS) to those two targets.

Tested by building it on a system that depends on LDFLAGS to produce
working binaries and confirming that `make check` succeeds.

Reported-by: Eric Christopher <echristo@google.com>
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>