Move configuration into a dedicated TestConfig struct.

This removes some duplicate code in parsing command-line flags and, more
importantly, makes configuration available when constructing the SSL_CTX and
avoids a number of globals.

Change-Id: I26e2d2285b732f855a2c82752bc8e0db480c3b30
Reviewed-on: https://boringssl-review.googlesource.com/1502
Reviewed-by: Adam Langley <agl@google.com>