scons: Change how the test object file suffix is applied.

This had been done by prepending the letter "t" to the suffix, with the
intention of turning a suffix like ".o" to ".to". Unfortunately SCons
stores both the actual suffix and the "." in that variable, so what we
ended up with was ".o" => "t.o", so test.o would become testt.o.

This change updates that logic to prepend a ".t" in front of the
existing suffix, skipping over it's first character which is assumed to
be a ".".

Change-Id: Id8c5f893413284868c2dc2a1a5e879b86790ed76
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50067
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -541,8 +541,8 @@ class GTest(Executable):
     @classmethod
     def declare_all(cls, env):
         env = env.Clone()
-        env['OBJSUFFIX'] = 't' + env['OBJSUFFIX']
-        env['SHOBJSUFFIX'] = 't' + env['SHOBJSUFFIX']
+        env['OBJSUFFIX'] = '.t' + env['OBJSUFFIX'][1:]
+        env['SHOBJSUFFIX'] = '.t' + env['SHOBJSUFFIX'][1:]
         env.Append(LIBS=env['GTEST_LIBS'])
         env.Append(CPPFLAGS=env['GTEST_CPPFLAGS'])
         env['GTEST_LIB_SOURCES'] = Source.all.with_tag(env, 'gtest lib')