Ignore the threadstest_fips executable

Add a new executable that isn't caught by existing .gitignore rules

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15453)

.gitignore

@@ -100,6 +100,7 @@ providers/common/include/prov/der_sm2.h
 /test/confdump
 /test/bio_prefix_text
 /test/evp_extra_test2
+/test/threadstest_fips
 
 # Certain files that get created by tests on the fly
 /test-runs