Tests: Move bigint tests to where BoringSSL puts them.

BoringSSL split up there bn_tests.txt into multiple files, which we had
done previously. Prepare to merge that BoringSSL change by putting the
test input files in the same places.

src/arithmetic/bigint.rs

@@ -846,7 +846,7 @@ mod tests {
     fn test_elem_exp_consttime() {
         let cpu_features = cpu::features();
         test::run(
-            test_file!("bigint_elem_exp_consttime_tests.txt"),
+            test_file!("../../crypto/fipsmodule/bn/test/mod_exp_tests.txt"),
             |section, test_case| {
                 assert_eq!(section, "");
 
@@ -875,7 +875,7 @@ mod tests {
     fn test_elem_mul() {
         let cpu_features = cpu::features();
         test::run(
-            test_file!("bigint_elem_mul_tests.txt"),
+            test_file!("../../crypto/fipsmodule/bn/test/mod_mul_tests.txt"),
             |section, test_case| {
                 assert_eq!(section, "");