Tests: Fix build on macOS without command line tools installed

Overwriting QMAKE_CXXFLAGS will remove the needed isysroot flag
for finding the standard headers.
Explicitly settings the standard lib on macOS is also no longer
necessary.

Change-Id: I8a5ae5df485785b473a435c7feb9f8fb3295952b
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/auto/valgrind/memcheck/testapps/overlap/overlap.pro

@@ -3,7 +3,7 @@ TARGET = overlap
 
 CONFIG += debug console
 CONFIG -= qt
-!msvc: QMAKE_CXXFLAGS = -O0 -fno-builtin
+!msvc: QMAKE_CXXFLAGS *= -O0 -fno-builtin
 
 macx:CONFIG -= app_bundle
 

tests/unit/echoserver/echoserver.pro

@@ -8,8 +8,6 @@ TEMPLATE = app
 
 unix:LIBS += -ldl
 
-osx:QMAKE_CXXFLAGS = -stdlib=libc++
-
 include(../../../qtcreator.pri)
 include(../../../src/libs/clangbackendipc/clangbackendipc-lib.pri)
 include(../../../src/libs/sqlite/sqlite-lib.pri)

tests/unit/unittest/unittest.pro

@@ -11,8 +11,6 @@ include(benchmark_dependency.pri)
 
 OBJECTS_DIR = $$OUT_PWD/obj # workaround for qmake bug in object_parallel_to_source
 
-osx:QMAKE_CXXFLAGS = -stdlib=libc++
-
 force_debug_info:QMAKE_CXXFLAGS += -fno-omit-frame-pointer
 
 DEFINES += \