[gdb/testsuite] Minimize gdb restarts

Minimize gdb restarts, applying the following rules:
- don't use prepare_for_testing unless necessary
- don't use clean_restart unless necessary

Also, if possible, replace build_for_executable + clean_restart
with prepare_for_testing for brevity.

Touches 68 test-cases.

Tested on x86_64-linux.

gdb/testsuite/gdb.python/py-infthread.exp

@@ -24,9 +24,6 @@ if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
     return -1
 }
 
-# Start with a fresh gdb.
-clean_restart ${testfile}
-
 # Skip all tests if Python scripting is not enabled.
 if { [skip_python_tests] } { continue }