Add ISM1 DTX testcase to self_test.prm

Currently, there is no testcase in the self test for ISM1 DTX. Since there is work going on with ISM and DTX, it would be good to have at least one testcase for this in the MR pipeline.

Using the available testvector stv1ISM48s.pcm, no dtx is triggered (tested by running IVAS_cod -dtx -ism 1 scripts/testv/stvISM1.csv 13200 48 scripts/testv/stv1ISM48s.pcm bit in the debugger and never hitting a breakpoint at ivas_ism_enc.c:214). However, when using the mono testvector stv48n.pcm instead, dtx is triggered (tested with same method). In conjunction with adding DTX support for more ISM operating points and the coverage analysis, we will probably have to revise testvectors and test cases soon again, but as an intermediate version, I would add one testcase with -dtx using the mentioned mono testvector to have at least a bit of testing for ISM DTX ongoing.