test: flush stdout after forking

meson test was not capturing the intended output from the child
process; force a flush to ensure it reaches the test log.

Signed-off-by: John Levon <john.levon@nutanix.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

app/test/process.h

@@ -110,6 +110,7 @@ process_dup(const char *const argv[], int numargs, const char *env_value)
 		for (i = 0; i < num; i++)
 			printf("'%s' ", argv_cpy[i]);
 		printf("\n");
+		fflush(stdout);
 
 		/* set the environment variable */
 		if (setenv(RECURSIVE_ENV_VAR, env_value, 1) != 0)