Fix the build timeout when 'leak' profile is active

Motivation:

AbstractByteBufTest.testInternalBuffer() uses writeByte() operations to
populate the sample data.  Usually, this isn't a problem, but it starts
to take a lot of time when the resource leak detection level gets
higher.

In our CI machine, testInternalBuffer() takes more than 30 minutes,
causing the build timeout when the 'leak' profile is active (paranoid
level resource detection.)

Modification:

Populate the sample data using ThreadLocalRandom.nextBytes() instead of
using millions of writeByte() operations.

Result:

Test runs much faster when leak detection level is high.