Merge #21244: Move GetDataDir to ArgsManager

bb8d1c6e02 Change ClearDataDirPathCache() to ArgsManager.ClearPathCache(). (Kiminuo)
b4190eff72 Change GetBlocksDir() to ArgsManager.GetBlocksDirPath(). (Kiminuo)
83292e2a70 scripted-diff: Modify unit tests to use the ArgsManager in the BasicTestingSetup class instead of implicitly relying on gArgs. (Kiminuo)
55c68e6f01 scripted-diff: Replace m_args with m_local_args in getarg_tests.cpp (Kiminuo)
511ce3a26b BasicTestingSetup: Add ArgsManager. (Kiminuo)
1cb52ba065 Modify "util_datadir" unit test to not use gArgs. (Kiminuo)
1add318704 Move GetDataDir(fNetSpecific) implementation to ArgsManager. (Kiminuo)
70cdf679f8 Move StripRedundantLastElementsOfPath before ArgsManager class. (Kiminuo)

Pull request description:

  This PR attempts to contribute to "Remove gArgs" (#21005).

  Main changes:

  * `GetDataDir()` function is moved to `ArgsManager.GetDataDirPath()`.
  * `GetBlocksDir()` function is moved to `ArgsManager.GetBlocksDirPath()`.

ACKs for top commit:
  ryanofsky:
    Code review ACK bb8d1c6e02. Just minor const/naming changes and splitting/scripting commits since last review
  MarcoFalke:
    review ACK bb8d1c6e02 📓
  hebasto:
    re-ACK bb8d1c6e02, addressed comments, and two commits made scripted-diffs since my [previous](https://github.com/bitcoin/bitcoin/pull/21244#pullrequestreview-638270583) review.

Tree-SHA512: ba9408c22129d6572beaa103dca0324131766f06d562bb7d6b9e214a0a4d40b0216ce861384562bde24b744003b3fbe6fac239061c8fd798abd3981ebc1b9019

src/qt/clientmodel.cpp

@@ -216,7 +216,7 @@ QString ClientModel::dataDir() const
 
 QString ClientModel::blocksDir() const
 {
-    return GUIUtil::boostPathToQString(GetBlocksDir());
+    return GUIUtil::boostPathToQString(gArgs.GetBlocksDirPath());
 }
 
 void ClientModel::updateBanlist()