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>