testsuite: run mpiexec.hydra -launcher fork

Problem: occasionally mpiexec output from spawned tasks
is lost, causing test to fail sporadically.

Try adding the "-launcher fork" option.  This overrides
the default launcher, which is "ssh".

Maybe this will fix flux-framework#1169

t/t2004-hydra.t

@@ -14,35 +14,35 @@ if ! which mpiexec.hydra 2>/dev/null; then
 fi
 
 test_expect_success 'Hydra runs hello world' '
-	mpiexec.hydra -n 4 echo "Hello World"
+	mpiexec.hydra -launcher fork -n 4 echo "Hello World"
 '
 
 count_uniq_lines() { sort $1 | uniq | wc -l; }
 
 test_expect_success 'Hydra sets PMI_FD to unique value' '
-	mpiexec.hydra -n 4 printenv PMI_FD > out &&
+	mpiexec.hydra -launcher fork -n 4 printenv PMI_FD > out &&
 	test_debug "cat out" &&
 	test $(count_uniq_lines out) -eq 4
 '
 
 test_expect_success 'Hydra sets PMI_RANK to unique value' '
-	mpiexec.hydra -n 4 printenv PMI_RANK > out2 &&
+	mpiexec.hydra -launcher fork -n 4 printenv PMI_RANK > out2 &&
 	test_debug "cat out2" &&
 	test $(count_uniq_lines out2) -eq 4
 '
 
 test_expect_success 'Hydra sets PMI_SIZE to uniform value' '
-	mpiexec.hydra -n 4 printenv PMI_SIZE > out3 &&
+	mpiexec.hydra -launcher fork -n 4 printenv PMI_SIZE > out3 &&
 	test_debug "cat out3" &&
 	test $(count_uniq_lines out3) -eq 1
 '
 
 test_expect_success 'Flux libpmi-client wire protocol works with Hydra' '
-	mpiexec.hydra -n 4 ${PMINFO}
+	mpiexec.hydra -launcher fork -n 4 ${PMINFO}
 '
 
 test_expect_success 'Hydra can launch Flux' '
-	mpiexec.hydra -n 4 flux start \
+	mpiexec.hydra -launcher fork -n 4 flux start \
 		flux comms info >flux_out &&
 	test_debug "cat flux_out" &&
 	grep size=4 flux_out