Ensure that tests reads tigrc from the default location
Extend tigrc parse test to check parsed bindings via the help view
Add test for TIGRC_USER and TIGRC_SYSTEM env variables

This also fixes a segfault when TERM=dumb due to built-in line rules not
being properly initialized.
Document the test framework and improve the test output

This adds the possibility to configure tests via TEST_OPTS env variable
which is used to control the indentation depending on whether showing
verbose output or not.