[CI] Run leveldb tests independently of other builds and tests

Summary:
This is expected to fix some occasional CI failures such as
https://build.bitcoinabc.org/viewLog.html?buildId=29713.

Test Plan: See this diff build result on CI.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5317