tests/: move tests around

diff --git a/tests/src/test-fft.c b/tests/src/spectral/test-fft.c
similarity index 100%
rename from tests/src/test-fft.c
rename to tests/src/spectral/test-fft.c

diff --git a/tests/src/test-beattracking.c b/tests/src/tempo/test-beattracking.c
similarity index 100%
rename from tests/src/test-beattracking.c
rename to tests/src/tempo/test-beattracking.c

diff --git a/tests/src/test-tempo.c b/tests/src/tempo/test-tempo.c
similarity index 100%
rename from tests/src/test-tempo.c
rename to tests/src/tempo/test-tempo.c