demo: Add test to verify workload is running with --with-load

Adds a test ensuring the workload starts up when running
`cockroach demo movr --with-load`

Release note: None

pkg/cli/interactive_tests/test_demo_workload.tcl

@@ -0,0 +1,24 @@
+#! /usr/bin/env expect -f
+
+source [file join [file dirname $argv0] common.tcl]
+
+start_test "Check cockroach demo --with-load runs the movr workload"
+
+# Start demo with movr and the movr workload.
+spawn $argv demo movr --with-load
+
+eexpect "movr>"
+
+# Try a few times, but expect that we eventually see the workload
+# queries show up as the highest count queries in the system.
+
+for {set i 0} {$i < 10} {incr i} {
+  send "select key from crdb_internal.node_statement_statistics order by count desc limit 1;\r"
+  # Sleep a little bit to let the workload catch up.
+  sleep 1
+}
+
+eexpect "SELECT city, id FROM vehicles WHERE city = \$1"
+
+interrupt
+end_test