make running the test suite slightly easier

- default to running all tests if no command-line arguments provided
- cd to the test directory, then back when finished
- use at most half of the cpu cores when no command-line arguments given
add Base.runtests()
add test list and numcores optional inputs to Base.runtests
allow giving a list of tests in one string

such as Base.runtests("file numbers parallel")
Merge pull request #7846 from tkelman/test-defaults

RFC: make running the test suite slightly easier