build(plugin-keychain-memory): fix local imports broken due to .js exts

Found the solution and a long discussion about it here:
kulshekhar/ts-jest#1057

TLDR: The Jest resolver needs a little extra information/tweak to the
config so that it can correctly handle the .js imports.

Specifically this comment provided the solution which I made here:
kulshekhar/ts-jest#1057 (comment)

Fixes hyperledger#3254

Signed-off-by: Peter Somogyvari <peter.somogyvari@accenture.com>