ci: tweak mpi setup to allow -s and hide output

conftest.py

@@ -5,7 +5,7 @@
 import pytest
 from sympy import Add
 
-from devito import Eq, configuration, Revolver  # noqa
+from devito import Eq, configuration, Revolver, switchconfig  # noqa
 from devito.checkpointing import NoopRevolver
 from devito.finite_differences.differentiable import EvalDerivative
 from devito.arch import Cpu64, Device, sniff_mpi_distro, Arm
@@ -141,17 +141,26 @@ def parallel(item, m):
         else:
             raise ValueError("Can't run test: unexpected mode `%s`" % m)
 
+    # Don't waste time with subprocess for single rank
+    if nprocs == 1:
+        with switchconfig(mpi=scheme):
+            try:
+                item.runtest()
+                return True
+            except:
+                return False
+
     pyversion = sys.executable
+    pyarg = item.config.invocation_params.args[0]
     # Only spew tracebacks on rank 0.
     # Run xfailing tests to ensure that errors are reported to calling process
     if item.cls is not None:
         testname = "%s::%s::%s" % (item.fspath, item.cls.__name__, item.name)
     else:
         testname = "%s::%s" % (item.fspath, item.name)
-    args = ["-n", "1", pyversion, "-m", "pytest", "--runxfail", "-q", testname]
-    if nprocs > 1:
-        args.extend([":", "-n", "%d" % (nprocs - 1), pyversion, "-m", "pytest",
-                     "--runxfail", "--tb=no", "-q", testname])
+
+    args = ["-n", str(nprocs), pyversion, "-m", "pytest", "--no-summary", pyarg,
+            "--runxfail", "-qq", testname]
     # OpenMPI requires an explicit flag for oversubscription. We need it as some
     # of the MPI tests will spawn lots of processes
     if mpi_distro == 'OpenMPI':

tests/test_linearize.py

@@ -170,7 +170,6 @@ def test_codegen_quality0(mode):
     # for the efunc args
     # (the other three obviously are _POSIX_C_SOURCE, START, STOP)
     assert len(op._headers) == 6
-    return "bonjour"
 
 
 def test_codegen_quality1():