Make tests follow Qt conventions by starting all unit tests with `tst_'.

Some scripts make use of this convention to differentiate unit tests
from applications which happen to link to testlib.

Reviewed-by: ossi

tests/auto/cplusplus/ast/ast.pro

@@ -3,3 +3,5 @@ CONFIG += qt warn_on console depend_includepath
 QT = core testlib
 include(../shared/shared.pri)
 SOURCES += tst_ast.cpp
+TARGET=tst_$$TARGET
+

tests/auto/cplusplus/lookup/lookup.pro

@@ -5,3 +5,4 @@ QT = core testlib
 include(../../../../src/libs/cplusplus/cplusplus-lib.pri)
 
 SOURCES += tst_lookup.cpp
+TARGET=tst_$$TARGET

tests/auto/cplusplus/semantic/semantic.pro

@@ -4,3 +4,4 @@ QT = core testlib
 include(../shared/shared.pri)
 
 SOURCES += tst_semantic.cpp
+TARGET=tst_$$TARGET