Test fix "too many open files" on macOS (#14853)

On macOS, running `make test` often fails with "too many open files" due
to the low default limit (usually 256).

This PR increases the limit by adding `ulimit -n 4096` so that the tests
have enough file descriptors for concurrent connections.

runtest

@@ -1,4 +1,9 @@
 #!/bin/sh
+# Raise open files limit (macOS default 256 is too low for tests).
+OPEN_FILE_LIMIT=$(ulimit -n 2>/dev/null)
+if [ -n "$OPEN_FILE_LIMIT" ] && [ "$OPEN_FILE_LIMIT" != "unlimited" ] && [ "$OPEN_FILE_LIMIT" -lt 1024 ]; then
+    ulimit -n 1024 2>/dev/null || true
+fi
 TCL_VERSIONS="8.5 8.6 8.7 9.0"
 TCLSH=""
 

runtest-cluster

@@ -1,4 +1,9 @@
 #!/bin/sh
+# Raise open files limit (macOS default 256 is too low for tests).
+OPEN_FILE_LIMIT=$(ulimit -n 2>/dev/null)
+if [ -n "$OPEN_FILE_LIMIT" ] && [ "$OPEN_FILE_LIMIT" != "unlimited" ] && [ "$OPEN_FILE_LIMIT" -lt 1024 ]; then
+    ulimit -n 1024 2>/dev/null || true
+fi
 TCL_VERSIONS="8.5 8.6 8.7 9.0"
 TCLSH=""
 

runtest-moduleapi

@@ -1,4 +1,9 @@
 #!/bin/sh
+# Raise open files limit (macOS default 256 is too low for tests).
+OPEN_FILE_LIMIT=$(ulimit -n 2>/dev/null)
+if [ -n "$OPEN_FILE_LIMIT" ] && [ "$OPEN_FILE_LIMIT" != "unlimited" ] && [ "$OPEN_FILE_LIMIT" -lt 1024 ]; then
+    ulimit -n 1024 2>/dev/null || true
+fi
 TCL_VERSIONS="8.5 8.6 8.7 9.0"
 TCLSH=""
 [ -z "$MAKE" ] && MAKE=make

runtest-sentinel

@@ -1,4 +1,9 @@
 #!/bin/sh
+# Raise open files limit (macOS default 256 is too low for tests).
+OPEN_FILE_LIMIT=$(ulimit -n 2>/dev/null)
+if [ -n "$OPEN_FILE_LIMIT" ] && [ "$OPEN_FILE_LIMIT" != "unlimited" ] && [ "$OPEN_FILE_LIMIT" -lt 1024 ]; then
+    ulimit -n 1024 2>/dev/null || true
+fi
 TCL_VERSIONS="8.5 8.6 8.7 9.0"
 TCLSH=""