Makefile: don't use srcdir in TESTS

src/Makefile-env.am:31: error: using '$(srcdir)' in TESTS is currently broken: '$(srcdir)/unittest_bufferlist.sh'
src/Makefile.am:1:   'src/Makefile-env.am' included from here
src/Makefile-env.am:31: error: using '$(srcdir)' in TESTS is currently broken: '$(srcdir)/test/encoding/check-generated.sh'
src/Makefile.am:1:   'src/Makefile-env.am' included from here

on my (raring) laptop.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #639 from ceph/wip-gtest

Makefile: don't use srcdir in TESTS

Reviewed-by: Roald J. van Loon <roaldvanloon@gmail.com>