debug: workaround for sporadic failures of some tests due to unexpect…

…ed data present in pexpect match

Problem was observed on UnavailableMultiTest - this test was sporadically failing.
When the failure was observed the log of the failing test looked as follows:

```
  File "/whatever/RISCVTests/debug/testlib.py", line 504, in <genexpr>
    if all(targets[hart.id]["State"] == "running" for hart in harts):
           ~~~~~~~~~~~~~~~~^^^^^^^^^
KeyError: 'State'
```

Adding this modification to testlib.py

```
--- a/debug/testlib.py
+++ b/debug/testlib.py
@@ -498,6 +498,10 @@ class Openocd:
         for line in lines[2:]:
             if line.strip():
                 data.append(dict(zip(headers, line.split()[1:])))
+        str_data = str(data)
+        sys.stdout.flush()
+        sys.stdout.write(f"parsed targets:\n{result}\n===\n{str_data}\n---\n")
+        sys.stdout.flush()
         return data
```

Allowed me to root cause the issue. Namely we have the following
situation:

```
parsed targets:
Exception ignored in: <function _TemporaryFileCloser.__del__ at 0x7f2dee64d1c0>
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/tempfile.py", line 450, in __del__
    self.close()
  File "/usr/local/lib/python3.11/tempfile.py", line 446, in close
    unlink(self.name)
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/[email protected]'
...
    TargetName         Type       Endian TapName            State
--  ------------------ ---------- ------ ------------------ ------------
 0  riscv.cpu0         riscv      little riscv.cpu          running
 1* riscv.cpu1         riscv      little riscv.cpu          running
===
[{'Exception': '"/usr/local/lib/python3.11/tempfile.py",', 'ignored': 'line', 'in:': '450,', ...
```

The only reasonable (to me) explanation for the observed behavior is below.

Here is how we connect to TCL-RPC server:

```
  self.openocd_cli = pexpect.spawn(f"nc localhost {self.tclrpc_port}")
  tty.setraw(self.openocd_cli.child_fd)
```

Later we request target list by issuing "targets" command:

```
  self.command("targets")
```

Internally, pexpect.spawn implemented as follows:

- we fork the process
- set up pty and then call execve
- all these steps are written in python

"Exception ignored" messages are result of exceptions thrown from
finalizers of NamedTemporaryFile objects. When exception is thrown from
the finalizer - python unconditionally prints a "warning" to stderr. It
seems that these messages are polluting our output from "nc" since python
GC can be invoked before the execve syscall.

The workaround is just to make sure that execve was executed before we
rely on the format of command output. To have such a guarantee we just
issue a dummy "echo" command and check that we have a proper reply in the
output stream.

While this explanation looks convincing, the behavior above still looks
strange, given that we have https://bugs.python.org/issue14548 which
was resolved long ago.

However, the proposed workaround fixes the issue.

debug/testlib.py

@@ -367,10 +367,15 @@ def __init__(self, server_cmd=None, config=None, debug=False, timeout=60,
         self.tclrpc_port = None
         self.start(cmd, logfile, extra_env)
 
-        self.openocd_cli = pexpect.spawn(f"nc localhost {self.tclrpc_port}")
+        self.openocd_cli = pexpect.spawn(f"nc localhost {self.tclrpc_port}",
+            echo=False)
         # TCL-RPC uses \x1a as a watermark for end of message. We set raw
         # pty mode to disable translation of \x1a to EOF
         tty.setraw(self.openocd_cli.child_fd)
+        hello_string = self.command(
+            "capture { echo \"Hello TCL-RPC!\" }").decode()
+        if not "Hello TCL-RPC!" in hello_string:
+            raise RuntimeError(f"TCL-RPC - unexpected reply:\n{hello_string}")
 
     def start(self, cmd, logfile, extra_env):
         combined_env = {**os.environ, **extra_env}