test: use `nextest` for running tests if present

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

.config/nextest.toml

@@ -0,0 +1,3 @@
+[profile.loom]
+# loom tests might take a long time; don't have `nextest` print "slow" messages.
+slow-timeout = "10m"

bin/loom

@@ -2,10 +2,17 @@
 
 set -x
 
+TESTCMD=(test)
+
+# if cargo nextest is installed, use it instead!
+if cargo list | grep -q "nextest"; then
+    TESTCMD=(nextest run --cargo-profile loom)
+fi
+
 RUSTFLAGS="--cfg loom ${RUSTFLAGS}" \
 LOOM_LOG="${LOOM_LOG:-info}" \
 LOOM_MAX_PREEMPTIONS="${LOOM_MAX_PREEMPTIONS:-2}" \
-cargo test \
+cargo ${TESTCMD[@]} \
     --profile loom \
     --lib \
     "$@"