Fix make check.
Fix make check.

The make variable TESTS is reserved by automake:
https://www.gnu.org/software/automake/manual/automake.html#index-TESTS-832
and thus it causes error when make check:
 make[4]: Entering directory `/home/ueno/devel/ibus/src'
 /bin/sh: line 9: ./tests: Is a directory
 FAIL: tests

BUG=none
TEST=manually

Review URL: https://codereview.appspot.com/6056043

fujiwarat/ibus,fujiwarat/ibus,j717273419/ibus,ibus/ibus,Keruspe/ibus,ueno/ibus,ueno/ibus,fujiwarat/ibus,Keruspe/ibus,j717273419/ibus,ibus/ibus,Keruspe/ibus,fujiwarat/ibus,j717273419/ibus,ueno/ibus,ibus/ibus,ueno/ibus,Keruspe/ibus,j717273419/ibus,ueno/ibus,ibus/ibus