[tests] simplify bts directory as well

src/plugins/e-acsl/tests/bts/bts1304.i

src/plugins/e-acsl/tests/bts/bts1307.i

src/plugins/e-acsl/tests/bts/bts1324.i

src/plugins/e-acsl/tests/bts/bts1326.i

src/plugins/e-acsl/tests/bts/bts1390.c

src/plugins/e-acsl/tests/bts/bts1398.c

src/plugins/e-acsl/tests/bts/bts1399.c

src/plugins/e-acsl/tests/bts/bts1478.c

src/plugins/e-acsl/tests/bts/bts1700.i

src/plugins/e-acsl/tests/bts/bts1717.i

src/plugins/e-acsl/tests/bts/bts1718.i

src/plugins/e-acsl/tests/bts/bts1837.i

src/plugins/e-acsl/tests/bts/oracle/bts1304.0.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1304.1.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1304.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1307.0.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1307.1.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1307.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1324.1.res.oracle

src/plugins/e-acsl/tests/bts/oracle/bts1324.err.oracle