Fix memory leak in x509_req_test

Fix memory leak in x509_req_test

Running the x509_req_test with address sanitizer shows a memory leak:

==186455==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 53 byte(s) in 1 object(s) allocated from:
    #0 0x3ffad5f47af in malloc (/lib64/libasan.so.8+0xf47af) (BuildId: 93b3d2536d76f772a95880d76c746c150daabbee)
    #1 0x3ffac4214fb in CRYPTO_malloc crypto/mem.c:202
    #2 0x3ffac421759 in CRYPTO_zalloc crypto/mem.c:222
    #3 0x100e58f in test_mk_file_path test/testutil/driver.c:450
    #4 0x1004671 in test_x509_req_detect_invalid_version test/x509_req_test.c:32
    #5 0x100d247 in run_tests test/testutil/driver.c:342
    #6 0x10042e3 in main test/testutil/main.c:31
    #7 0x3ffaad34a5b in __libc_start_call_main (/lib64/libc.so.6+0x34a5b) (BuildId: 461b58df774538594b6173825bed67a9247a014d)
    #8 0x3ffaad34b5d in __libc_start_main@GLIBC_2.2 (/lib64/libc.so.6+0x34b5d) (BuildId: 461b58df774538594b6173825bed67a9247a014d)
    #9 0x1004569  (/root/openssl/test/x509_req_test+0x1004569) (BuildId: ab6bce0e531df1e3626a8f506d07f6ad7c7c6d57)
SUMMARY: AddressSanitizer: 53 byte(s) leaked in 1 allocation(s).

The certFilePath that is obtained via test_mk_file_path() must be freed when
no longer used.

While at it, make the certFilePath variable a local variable, there is no need
to have this a global static variable.

Fixes: https://github.com/openssl/openssl/commit/7d2c0a4b1feb152ee1190dfedc65dfd1c928f9e5
Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24715)