tests: revisit spdx.is_expression()

[jkowalleck]

closes #781

[jkowalleck]

resear chresilts:

some parts of the SPDX expression language are not supported by the utilized 3rd-party library

• caused SPDX validation should consider LicenseRef-* that are not in the database aboutcode-org/license-expression#109
• caused SPDX validation should consider arbitrary expressions in format <KNOWN-LICENSE-ID>"+" as valid aboutcode-org/license-expression#110

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ +0.00%	✅ ∅ (target: 80.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (8546fb6)	5187	4855	93.60%
Head commit (a9bf54b)	5187 (+0)	4855 (+0)	93.60% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#782)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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