test/test-lib.sh: execute basename(1)s lazier in test_expect_equal_file ()

Moved the 2 basename(1) executions to the test failure branch in
test_expect_equal_file ().

The output of basename(1) executions in function test_expect_equal_file ()
are only used when tests fails -- when all tests pass these 2 basename(1)
executions are no longer done at all.