testsc.c: fix further memory leaks.

These were spotted by Leak Sanitiser rather than Coverity: it reported
them while I was checking the fixes for Coverity-spotted issues.

testsc.c

@@ -334,6 +334,7 @@ static void test_mp_get_nbits(void)
         log_end();
     }
     mp_free(prev);
+    mp_free(z);
 }
 
 static void test_mp_from_decimal(void)
@@ -386,6 +387,7 @@ static void test_mp_string_format(char *(*mp_format)(mp_int *x))
         log_end();
         sfree(formatted);
     }
+    mp_free(z);
 }
 
 static void test_mp_get_decimal(void)
@@ -589,6 +591,10 @@ static void test_mp_modarith(
 
         mp_free(out);
     }
+
+    mp_free(base);
+    mp_free(exponent);
+    mp_free(modulus);
 }
 
 static void test_mp_modadd(void)
@@ -625,6 +631,8 @@ static void test_mp_invert_mod_2to(void)
 
         mp_free(out);
     }
+
+    mp_free(x);
 }
 
 static void test_mp_modsqrt(void)
@@ -647,7 +655,8 @@ static void test_mp_modsqrt(void)
 
     /* Do one initial call to cause the lazily initialised sub-context
      * to be set up. This will take a while, but it can't be helped. */
-    mp_modsqrt(sc, x, &success);
+    mp_int *unwanted = mp_modsqrt(sc, x, &success);
+    mp_free(unwanted);
 
     for (size_t i = 0; i < looplimit(8); i++) {
         mp_random_bits_into(x, bits - 1);
@@ -802,6 +811,7 @@ static void test_ecc_weierstrass_multiply(void)
     }
     ecc_weierstrass_point_free(a);
     ecc_weierstrass_curve_free(wc);
+    mp_free(exponent);
 }
 
 static void test_ecc_weierstrass_is_identity(void)
@@ -1000,6 +1010,7 @@ static void test_ecc_montgomery_multiply(void)
     }
     ecc_montgomery_point_free(a);
     ecc_montgomery_curve_free(wc);
+    mp_free(exponent);
 }
 
 static void test_ecc_montgomery_get_affine(void)
@@ -1132,6 +1143,7 @@ static void test_ecc_edwards_multiply(void)
     }
     ecc_edwards_point_free(a);
     ecc_edwards_curve_free(ec);
+    mp_free(exponent);
 }
 
 static void test_ecc_edwards_eq(void)