wip: eskip: add more comment test cases

[AlexanderYastrebov]

Followup on #2849

[AlexanderYastrebov]

Its not clear whether we want to support comments or multiline strings for ParseFilters and ParsePredicates helpers.

[AlexanderYastrebov]

Its tricky to support all test cases consistently due to how ParseFilters (and ParsePredicates fro that matter) is implemented:

rs, err := parse("* -> " + f + " -> <shunt>")

We can make comments on a separate line work by adding a newline:

rs, err := parse("* -> " + f + "\n -> <shunt>")

but its hard to make only comment input work:

rs, err := parse("* -> " + "// a comment" + "\n -> <shunt>")

because it produces invalid eskip.

We can not prohibit comments easily because they are skipped by lexer - the lexer would need to be changed to support "allow comments" flag.

Note also that due to use of "* -> " prefix parse errors contain incorrect token offset value.

The proper fix would require:

• separate parse methods for routes, predicates and filters (see https://www.gnu.org/software/bison/manual/html_node/Multiple-start_002dsymbols.html)
• a lexer support to prohibit comments for ParseFilters and ParsePredicates if we decide so

[AlexanderYastrebov]

Created a branch that separates document, predicates and filters parsing https://github.com/zalando/skipper/tree/eskip/separate-parse-predicates-filters
We can also disallow comments for ParsePredicates and ParseFilters, needs decision.

It includes several other changes that I'd like to land first before opening a PR.
Using the same approach of separate start symbols we can add singular ParseFilter and ParsePredicate for use in RouteGroup dataclient/validator instead of checking that ParseFilters returned a single result:

skipper/dataclients/kubernetes/definitions/routegroupvalidator.go

Lines 69 to 74 in e63fe2c

	filters, err := eskip.ParseFilters(f)
	if err != nil {
	errs = append(errs, err)
	} else if len(filters) != 1 {
	errs = append(errs, fmt.Errorf("%w at %q", errSingleFilterExpected, f))
	}

[AlexanderYastrebov]

Created draft PR #2873

[AlexanderYastrebov]

Should be rebased after #2874 gets merged.

[AlexanderYastrebov]

I've added these testcases to #2873