scons: Use a different suffix for test object files.

These files are built with a different command line, and so should be
distinct build artifacts in the build directory.

Change-Id: Iec9403ad73fbdbcb1cd268d68716c3aa85a80b24
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48137
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/SConscript

@@ -556,6 +556,8 @@ class GTest(Executable):
     @classmethod
     def declare_all(cls, env):
         env = env.Clone()
+        env['OBJSUFFIX'] = 't' + env['OBJSUFFIX']
+        env['SHOBJSUFFIX'] = 't' + env['SHOBJSUFFIX']
         env.Append(LIBS=env['GTEST_LIBS'])
         env.Append(CPPFLAGS=env['GTEST_CPPFLAGS'])
         env['GTEST_LIB_SOURCES'] = Source.all.with_tag(env, 'gtest lib')