Add tests for verify_restartable()

For selection of test cases, see comments added in the commit.

It makes the most sense to test with chains using ECC only, so for the chain
of length 2 we use server10 -> int-ca3 -> int-ca2 and trust int-ca2 directly.

Note: server10.crt was created by copying server10_int3_int-ca2.crt and
manually truncating it to remove the intermediates. That base can now be used
to create derived certs (without or with a chain) in a programmatic way.

library/x509_crt.c

@@ -2069,6 +2069,16 @@ static int x509_crt_check_ee_locally_trusted(
  * - EE, Ci1, ..., Ciq cannot be continued with a trusted root
  *   -> return that chain with NOT_TRUSTED set on Ciq
  *
+ * Tests for (aspects of) this function should include at least:
+ * - trusted EE
+ * - EE -> trusted root
+ * - EE -> intermedate CA -> trusted root
+ * - if relevant: EE untrusted
+ * - if relevant: EE -> intermediate, untrusted
+ * with the aspect under test checked at each relevant level (EE, int, root).
+ * For some aspects longer chains are required, but usually length 2 is
+ * enough (but length 1 is not in general).
+ *
  * Arguments:
  *  - [in] crt: the cert list EE, C1, ..., Cn
  *  - [in] trust_ca: the trusted list R1, ..., Rp