[tests] add startup delay to prevent expect desync (openthread#13174)

This commit adds a brief 0.1-second sleep delay immediately after
spawning node processes (rcp, cli, and mtd types) in the expect test
harness.

Under high CPU load on GitHub Actions runner VMs, the PTY file
descriptors can take a fraction of a second to fully initialize. If
commands are sent immediately after spawn without delay, the initial
expect match can fail with an instant timeout. This triggers duplicate
retransmissions in wait_for, leaving extra "Done" strings in expect's
PTY read buffer. The leftover "Done" strings desynchronize subsequent
assertions, causing tests to match cached output instead of waiting
for actual command execution (e.g., sending "diag stats" during an
active "diag send" command, which fails).

Adding a 100ms delay gives the PTY and child process enough time to
fully initialize and stabilize, avoiding instant timeouts and
subsequent test harness desynchronization.

Author: jwhui

tests/scripts/expect/_common.exp

@@ -118,6 +118,7 @@ proc spawn_node {id {type ""} {args ""}} {
             # avoid the error: "bind(sTxFd): Address already in use"
             sleep 0.2
             spawn /usr/bin/env GCOV_PREFIX=$gcov_prefix $ot_posix_apps/ot-cli $radio_url
+            sleep 0.1
             send "factoryreset\n"
             wait_for "state" "disabled"
             expect_line "Done"
@@ -127,6 +128,7 @@ proc spawn_node {id {type ""} {args ""}} {
         cli {
             spawn /usr/bin/env GCOV_PREFIX=$gcov_prefix $ot_simulation_apps/cli/ot-cli-ftd \
                 -L$ot_simulation_local_host $id
+            sleep 0.1
             send "factoryreset\n"
             wait_for "state" "disabled"
             expect_line "Done"
@@ -136,6 +138,7 @@ proc spawn_node {id {type ""} {args ""}} {
         mtd {
             spawn /usr/bin/env GCOV_PREFIX=$gcov_prefix $ot_simulation_apps/cli/ot-cli-mtd \
                 -L$ot_simulation_local_host $id
+            sleep 0.1
             send "factoryreset\n"
             wait_for "state" "disabled"
             expect_line "Done"