fix(esp_modem): DTE command race of timeout vs reply's signal

Race condtion:
* First command timeouted, but the reply came just after evaluation and
set signal variable to GOT_LINE
* Second command should timeout too, but a consistency check validates
that it timeouted and at the same time GOT_LINE (from previous step) and
throws an exception

STR:
* Revert change in esp_modem_dte.cpp
* Run TEST_CASE("DTE command races", "[esp_modem]")

Closes https://github.com/espressif/esp-protocols/issues/110

components/esp_modem/src/esp_modem_dte.cpp

@@ -36,6 +36,7 @@ command_result DTE::command(const std::string &command, got_line_cb got_line, ui
 {
     Scoped<Lock> l(internal_lock);
     command_result res = command_result::TIMEOUT;
+    signal.clear(GOT_LINE);
     command_term->set_read_cb([&](uint8_t *data, size_t len) {
         if (!data) {
             data = buffer.get();