tests/qtest/libqtest: Use GLib functions for proper const correctness

While commit e68da5b7a2cd ("tests/qtest: fix discarded const qualifier
warning") addressed the immediate strstr() warning by making 'found'
const, there's still a room for improvement: getenv() returns char *, but
environment strings are semantically read-only and should be treated as const
throughout their lifetime.

Replace getenv() with g_getenv() and strstr() with g_strstr_len() to
maintain const correctness from source to use. This approach:

- Uses g_getenv() which returns const gchar *, matching the read-only
  semantics of environment variables
- Employs g_strstr_len() for consistent use of GLib string functions,
  aligning with QEMU conventions
- Eliminates all const-correctness warnings with strict compilers

Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
Reviewed-by: Aditya Gupta <adityag@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>