Add string comparison tests.

Use computed string to exercise string interning code better.
Count individual testcases when running tests.

The idea is simple: each print represents a testcase within a test unit.
As we don't have strict rules on which/how many testcase are put into
a test file, it's nice to have an idea how many *testcases* we have
totally. Would be nice to count how many testcases pass/fail, but
that's a bit less trivial.