Rename old GC JTreg tests to the new naming scheme

At one point in time the convention was to put JTreg tests in a directory named after the bug number.

For many years now we've instead had the convention to use the @bug tag to identify the bug number and use the directory naming to group tests in a more logical manner.

The following tests should be moved out of the "bug number directories" and put in understandably named directories:

test/gc/7072527
test/gc/6845368
test/gc/6581734