Use wolfSSL 5.4.0 for tests

Use wolfSSL 5.4.0 for tests

The 5.4.0 update changed the default bignum implementation to what
could explicitly be enabled via `--enable-sp-math-all`.  Since this uses
fixed-sized buffers sufficient for key sizes of SP_INT_BITS, with a default
of 4096, modp6144 and modp8192 didn't work anymore (wc_DhGenerateKeyPair()
returned MP_EXPTMOD_E).  So we have to adapt the feature checks for this.

To support the larger DH groups we can either increase the buffer size
via `--with-max-rsa-bits` or add `--enable-heapmath` so buffers get
(re-)allocated as needed.  We go with the latter for now.