test: subsys: jwt: fix testing of the features

- Some files (PEM/DER files, jwt-test-cert.c, user-tls-conf.h)
  were removed because they are not used or no longer necessary.
  Private keys used in the test are now in the jwt-test-private.c
  file as arrays.
- testcase.yaml has been improved in order to test all possible
  use cases: ECDSA with TC, ECDSA with PSA, RSA;
- unnecessary Kconfigs were removed from prj.conf.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>