Run tests twice by default, first time deleting .pyc/.pyo files.

New option "-q" to leave .pyc/.pyo alone.

PCbuild/rt.bat

 @rem Run Tests.  Run the regression test suite.
 @rem Plain "rt" runs Release build, arguments passed on to regrtest.
 @rem "rt -d" runs Debug build similarly, after shifting off -d.
+@rem Normally the tests are run twice, the first time after deleting
+@rem all the .py[co] files from Lib/ and Lib/test.  But
+@rem "rt -q" (for Quick) runs the tests just once, and without
+@rem bothering to delete .py[co] files.
 @set _exe=python
-@if "%1" =="-d" set _exe=python_d
-@if "%1" =="-d" shift
+@set _qmode=no
+@if "%1"=="-q" set _qmode=yes
+@if "%1"=="-q" shift
+@if "%1"=="-d" set _exe=python_d
+@if "%1"=="-d" shift
+@if "%_qmode%"=="yes" goto LeavePyc
+@if "%1"=="-q" set _qmode=yes
+@if "%1"=="-q" shift
+@if "%_qmode%"=="yes" goto LeavePyc
+@echo Deleting .pyc/.pyo files ...
 @del ..\Lib\*.pyc
 @del ..\Lib\*.pyo
 @del ..\Lib\test\*.pyc
 @del ..\Lib\test\*.pyo
+:LeavePyc
 %_exe% ../lib/test/regrtest.py %1 %2 %3 %4 %5 %6 %7 %8 %9
+@if "%_qmode%"=="yes" goto Done
+@echo Running again without deleting .pyc/.pyo first:
+%_exe% ../lib/test/regrtest.py %1 %2 %3 %4 %5 %6 %7 %8 %9
+:Done
 @set _exe=
+@set _qmode=