Create tests for line-symbolizer, using expressions for parameters

- there are two versions:
  - line-symbolizer-expressions: stroke, stroke-width, stroke-opacity, offset
  - line-symbolizer-expressions-all: additionally test
    stroke-dasharray, stroke-linecap, stroke-linejoin, comp-op
    which are not supported right now
Added grid-reference.json files missing in 79fde58c0e1037e2494f5750eff861a51e70369e
Create tests for polygon-symbolizer, using expressions for parameters

- there are two versions:
  - polygon-symbolizer-expressions: fill, fill-opacity, gamma
  - polygon-symbolizer-expressions-all: additionally test
    comp-op which is not supported right now
Create tests for point-symbolizer, using expressions for parameters

- there are two versions:
  - point-symbolizer-expressions: file
  - point-symbolizer-expressions-all: additionally test
    allow-overlap, opacity, transform, ignore-placement, comp-op
    which are not supported right now
Merge pull request #2155 from plepe/expr-v2-tests

expr-v2: Create tests for symbolizers, using expressions for parameters