Merge #17015: qa: Explain QT_QPA_PLATFORM for gui tests

fa06bb6 qa: Do not force overwrite of QT_QPA_PLATFORM on windows for gui tests (MarcoFalke)
faccf5f doc: Explain QT_QPA_PLATFORM for gui tests (MarcoFalke)

Pull request description:

  Closes #17013

ACKs for top commit:
  promag:
    ACK fa06bb6.
  jonasschnelli:
    ACK fa06bb6
  ryanofsky:
    utACK fa06bb6
  fanquake:
    ACK fa06bb6 - tested on macOS using `QT_QPA_PLATFORM=cocoa src/qt/test/test_bitcoin-qt`.

Tree-SHA512: f257159f6e66b2df7e870ac832ae9ef09eea173c8b7cd766458f87cf22f94681c81dcc54dea030dbc97eab5e3ae5132a4ffe8a343431a4e40f7ee29dc808dcb1

Author: fanquake

src/qt/test/addressbooktests.cpp

@@ -146,7 +146,7 @@ void AddressBookTests::addressBookTests()
         // and fails to handle returned nulls
         // (https://bugreports.qt.io/browse/QTBUG-49686).
         QWARN("Skipping AddressBookTests on mac build with 'minimal' platform set due to Qt bugs. To run AppTests, invoke "
-              "with 'test_bitcoin-qt -platform cocoa' on mac, or else use a linux or windows build.");
+              "with 'QT_QPA_PLATFORM=cocoa test_bitcoin-qt' on mac, or else use a linux or windows build.");
         return;
     }
 #endif

src/qt/test/apptests.cpp

@@ -57,7 +57,7 @@ void AppTests::appTests()
         // and fails to handle returned nulls
         // (https://bugreports.qt.io/browse/QTBUG-49686).
         QWARN("Skipping AppTests on mac build with 'minimal' platform set due to Qt bugs. To run AppTests, invoke "
-              "with 'test_bitcoin-qt -platform cocoa' on mac, or else use a linux or windows build.");
+              "with 'QT_QPA_PLATFORM=cocoa test_bitcoin-qt' on mac, or else use a linux or windows build.");
         return;
     }
 #endif

src/qt/test/test_main.cpp

@@ -61,9 +61,9 @@ int main(int argc, char *argv[])
     // platform ("xcb", "windows", or "cocoa") so tests can't unintentionally
     // interfere with any background GUIs and don't require extra resources.
     #if defined(WIN32)
-        _putenv_s("QT_QPA_PLATFORM", "minimal");
+        if (getenv("QT_QPA_PLATFORM") == nullptr) _putenv_s("QT_QPA_PLATFORM", "minimal");
     #else
-        setenv("QT_QPA_PLATFORM", "minimal", 0);
+        setenv("QT_QPA_PLATFORM", "minimal", /* overwrite */ 0);
     #endif
 
     // Don't remove this, it's needed to access

src/qt/test/wallettests.cpp

@@ -123,9 +123,9 @@ void BumpFee(TransactionView& view, const uint256& txid, bool expectDisabled, st
 //
 // This also requires overriding the default minimal Qt platform:
 //
-//     src/qt/test/test_bitcoin-qt -platform xcb      # Linux
-//     src/qt/test/test_bitcoin-qt -platform windows  # Windows
-//     src/qt/test/test_bitcoin-qt -platform cocoa    # macOS
+//     QT_QPA_PLATFORM=xcb     src/qt/test/test_bitcoin-qt  # Linux
+//     QT_QPA_PLATFORM=windows src/qt/test/test_bitcoin-qt  # Windows
+//     QT_QPA_PLATFORM=cocoa   src/qt/test/test_bitcoin-qt  # macOS
 void TestGUI()
 {
     // Set up wallet and chain with 105 blocks (5 mature blocks for spending).
@@ -256,7 +256,7 @@ void WalletTests::walletTests()
         // and fails to handle returned nulls
         // (https://bugreports.qt.io/browse/QTBUG-49686).
         QWARN("Skipping WalletTests on mac build with 'minimal' platform set due to Qt bugs. To run AppTests, invoke "
-              "with 'test_bitcoin-qt -platform cocoa' on mac, or else use a linux or windows build.");
+              "with 'QT_QPA_PLATFORM=cocoa test_bitcoin-qt' on mac, or else use a linux or windows build.");
         return;
     }
 #endif