Set up testing infrastructure: adding external dependencies and

automating generation of test rules for Java tests.

GenTestRules.bzl copied from
https://gist.github.com/jart/9a9f48e216fd747275012d2f2364ac09. I
modified to make it recognize src/java/test/ as a prefix (in addition to
javatests/ and java/).

Bazel developer explained why this is optimal:
https://groups.google.com/d/msg/bazel-discuss/MFYYkJk1tFs/xltlv4m_AQAJ.

Change-Id: I8ff9d48b994efa9e7ed7a1e2c7b8afed8a05b94a
ORIGINAL_AUTHOR=Thai Duong <thaidn@gmail.com>

GitOrigin-RevId: 9abc2685664a9f85ceca6835827219531ac8c6c6