test: Emit a friendly error message if run with bash < 4.0.

test: Emit a friendly error message if run with bash < 4.0.

The bash code in the test suite is using associative arrays which were
only added to bash as of release 4.0.

If the test suite is run with an older bash, we now immediately error
out and explain the situation, (instead of emitting confusing error
messages and failing dozens of tests, which is what happened before
this change).