Fix OMP warnings in UMAP tests

Explicitly sets the threading layer for Numba to a simple implementation
that should always be available. UMAP uses Numba internally and, on some
systems, the Numba threading layer can get set to an OpenMP (OMP)
backend. Since Numba internally calls OMP with a function that is
deprecated, OMP emits a deprecation warning. This warning is especially
useless since we force UMAP to run with a single CPU. The main effect of
this warning was to break our functional tests.

For more details, see this Numba issue comment on GitHub:
numba/numba#5275 (comment)

tests/pathogen-embed-umap-with-distances.t

@@ -1,3 +1,7 @@
+Always use the simplest Numba threading layer, to avoid lower-level OMP warnings.
+
+  $ export NUMBA_THREADING_LAYER=workqueue
+
 Get a distance matrix from a H3N2 HA alignment.
 
   $ pathogen-distance \

tests/pathogen-embed-umap.t

@@ -1,3 +1,7 @@
+Always use the simplest Numba threading layer, to avoid lower-level OMP warnings.
+
+  $ export NUMBA_THREADING_LAYER=workqueue
+
 Run pathogen-embed with UMAP on a H3N2 HA alignment.
 
   $ pathogen-embed \