Rename some of the .spec files which test crypto (#3077)

* `matrix-client-crypto.spec.ts` only tested a very specific bit of crypto (olm
  encryption). It goes back to the very early days, before Megolm was invented.
  I've renamed it to `olm-encryption-spec.ts`.

* `megolm-integ.spec.ts` is more of a general crypto test; it was just called
  `megolm` to distinguish it from the Olm tests above. Renamed to
  `crypto.spec.ts`.

Author: richvdh

spec/integ/megolm-integ.spec.ts renamed to spec/integ/crypto.spec.ts

@@ -22,7 +22,7 @@ limitations under the License.
  *
  * Note that megolm (group) conversation is not tested here.
  *
- * See also `megolm.spec.js`.
+ * See also `crypto.spec.js`.
  */
 
 // load olm before the sdk if possible