[feat] Improve error message when parameters are undefined

[jjotero]

When a test with one or more undefined parameters was decorated, it the decorator would raise an error saying that the test is an abstract test. This error message might not be very clear to the user, so it is now changed to something more intuitive.

The "abstract test" nomenclature has been kept in the unit tests to keep things short, and a definition has also been added there for clarity.

[mabraham]

This would have helped me help myself better in #1887

[codecov-io]

Codecov Report

Merging #1888 (85dca12) into master (7eef1a8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1888   +/-   ##
=======================================
  Coverage   87.90%   87.90%           
=======================================
  Files          49       49           
  Lines        8451     8451           
=======================================
  Hits         7429     7429           
  Misses       1022     1022           

Impacted Files	Coverage Δ
reframe/core/meta.py	100.00% <ø> (ø)
reframe/core/decorators.py	97.05% <100.00%> (ø)

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