mtest: Connect /dev/null to stdin when not running in interactive mode

This allows tests to check whether stdin is a tty to figure out if they're
running in interactive mode or not.

It also makes sure that tests that are not running in interactive mode
don't inadvertendly try to read from stdin.

mesonbuild/mtest.py

@@ -1497,7 +1497,7 @@ class SingleTestRunner:
             await self._run_cmd(harness, cmd)
         return self.runobj
 
-    async def _run_subprocess(self, args: T.List[str], *,
+    async def _run_subprocess(self, args: T.List[str], *, stdin: T.Optional[int],
                               stdout: T.Optional[int], stderr: T.Optional[int],
                               env: T.Dict[str, str], cwd: T.Optional[str]) -> TestSubprocess:
         # Let gdb handle ^C instead of us
@@ -1523,6 +1523,7 @@ class SingleTestRunner:
                 signal.signal(signal.SIGINT, previous_sigint_handler)
 
         p = await asyncio.create_subprocess_exec(*args,
+                                                 stdin=stdin,
                                                  stdout=stdout,
                                                  stderr=stderr,
                                                  env=env,
@@ -1533,9 +1534,11 @@ class SingleTestRunner:
 
     async def _run_cmd(self, harness: 'TestHarness', cmd: T.List[str]) -> None:
         if self.console_mode is ConsoleUser.INTERACTIVE:
+            stdin = None
             stdout = None
             stderr = None
         else:
+            stdin = asyncio.subprocess.DEVNULL
             stdout = asyncio.subprocess.PIPE
             stderr = asyncio.subprocess.STDOUT \
                 if not self.options.split and not self.runobj.needs_parsing \
@@ -1549,6 +1552,7 @@ class SingleTestRunner:
             extra_cmd.append(f'--gtest_output=xml:{gtestname}.xml')
 
         p = await self._run_subprocess(cmd + extra_cmd,
+                                       stdin=stdin,
                                        stdout=stdout,
                                        stderr=stderr,
                                        env=self.runobj.env,