[libcalamares] Fix tests for umask != 022

- My usual test environment has umask set to 022, but on one dev
   box it is 002, leading to test failures (which show the test
   was bad, not that the umask-setting code is bad)

src/libcalamares/utils/Tests.cpp

@@ -155,8 +155,10 @@ LibCalamaresTests::testUmask()
     QTemporaryFile ft;
     QVERIFY( ft.open() );
 
+    // m gets the previous value of the mask (depends on the environment the
+    // test is run in, might be 002, might be 077), ..
     mode_t m = CalamaresUtils::setUMask( 022 );
-    QCOMPARE( CalamaresUtils::setUMask( m ), m );
+    QCOMPARE( CalamaresUtils::setUMask( m ), 022 );  // But now most recently set was 022
 
     for ( mode_t i = 0; i <= 0777 /* octal! */; ++i )
     {

src/libcalamares/utils/UMask.h

@@ -25,7 +25,10 @@
 
 namespace CalamaresUtils
 {
-/// @brief Wrapper for umask(2)
+/** @brief Wrapper for umask(2)
+ *
+ * Like umask(2), sets the umask and returns the previous value of the mask.
+ */
 DLLEXPORT mode_t setUMask( mode_t u );
 
 /** @brief RAII for setting and re-setting umask.