Declare in corresponding header files and dissolve main.h (#3796)

* Declare in corresponding header files and dissolve main.h

Apply include-what-you-use

* Remove further unncessary includes

* Incorporate review comment

* Do not declare static functions and variables in header

* Adapt test

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

test/tdb2_test.cpp

@@ -27,12 +27,10 @@
 #include <cmake.h>
 // cmake.h include header must come first
 
-#include <main.h>
-#include <stdlib.h>
 #include <test.h>
 #include <unistd.h>
 
-#include <iostream>
+#include "Context.h"
 
 namespace {