Support for advanced selection criteria for tests

[vkarak]

Currently, the selection criteria (tags and test names) can only by ANDed. We should add full flexibility for selecting tests. For example:

• I want tests with tags either production or maintenance
• I want all tests that their name starts with openfoam and their tag does not start with production.

Another requirement is that a subsequent selection expression should supersede any previously specified in the command-line. This should be useful with the execution modes.

Internal issue: https://madra.cscs.ch/scs/reframe/issues/344