Make a dummy PDB symbol server just for CI tests

[XVilka]

Every time CI runs a job it tries to perform a network request to fetch the PDB.
From time to time it fails. Or for some restricted workers, like Alpine Static it fails as well.
Idea is to implement simplest server just for testing purpose.

Ghidra does the same:

• https://github.com/NationalSecurityAgency/ghidra/blob/master/Ghidra/Features/PDB/src/test/java/pdb/symbolserver/DummySymbolServer.java
• https://github.com/NationalSecurityAgency/ghidra/blob/master/Ghidra/Features/PDB/src/test/java/pdb/symbolserver/HttpSymbolServerTest.java
• https://github.com/NationalSecurityAgency/ghidra/blob/master/Ghidra/Features/PDB/src/test/java/pdb/symbolserver/SymbolServerServiceTest.java

Moreover, the similar feature would be required to test debuginfod in the future: #1285

[stale]

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