minor: run macos CI tests single-threaded

Author: saghm

.evergreen/config.yml

@@ -127,7 +127,7 @@ functions:
           export SSL="${SSL}"
           . .evergreen/generate-uri.sh
 
-          ASYNC_RUNTIME=${ASYNC_RUNTIME} .evergreen/run-tests.sh
+          SINGLE_THREAD=${SINGLE_THREAD} ASYNC_RUNTIME=${ASYNC_RUNTIME} .evergreen/run-tests.sh
 
   "run atlas tests":
     - command: shell.exec
@@ -428,6 +428,8 @@ axes:
       - id: macos-10.14
         display_name: "MacOS 10.14"
         run_on: macos-1014
+        variables:
+          SINGLE_THREAD: true
 
 buildvariants:
 -

.evergreen/run-async-std-tests.sh

@@ -3,6 +3,13 @@
 set -o errexit
 
 . ~/.cargo/env
-RUST_BACKTRACE=1 cargo test --no-default-features --features async-std-runtime
-RUST_BACKTRACE=1 cargo test sync --no-default-features --features sync
-RUST_BACKTRACE=1 cargo test --doc sync --no-default-features --features sync
+
+if [ "$SINGLE_THREAD" = true ]; then
+	OPTIONS="-- --test-threads=1"
+fi
+
+echo "cargo test options: ${OPTIONS}"
+
+RUST_BACKTRACE=1 cargo test --no-default-features --features async-std-runtime $OPTIONS
+RUST_BACKTRACE=1 cargo test sync --no-default-features --features sync $OPTIONS
+RUST_BACKTRACE=1 cargo test --doc sync --no-default-features --features sync $OPTIONS

.evergreen/run-tokio-tests.sh

@@ -3,4 +3,11 @@
 set -o errexit
 
 . ~/.cargo/env
-RUST_BACKTRACE=1 cargo test
+
+if [ "$SINGLE_THREAD" = true ]; then
+	OPTIONS="-- --test-threads=1"
+fi
+
+echo "cargo test options: ${OPTIONS}"
+
+RUST_BACKTRACE=1 cargo test $OPTIONS