Silence Python invalid escape sequence warnings by turning some strings into raw strings

[illwieckz]

Silence such kind of warnings:

site_init:132: SyntaxWarning: invalid escape sequence '\$'
scons: Reading SConscript files ...
native_client/SConstruct:3626: SyntaxWarning: invalid escape sequence '\c'
  COVERAGE_ANALYZER='$COVERAGE_ANALYZER_DIR\coverage_analyzer.exe',
native_client/tools/command_tester.py:359: SyntaxWarning: invalid escape sequence '\*'
  match = re.match('\*\* (Signal|Mach exception) (\d+) from '
native_client/src/trusted/service_runtime/build.scons:827: SyntaxWarning: invalid escape sequence '\['
  RE_IDENT = '^\[[0-9,:.]*\] (e_ident\+1 = ELF)$'

It makes it easier to read error logs by not being confused by such warning noise.

[slipher]

I believe in this one, the backslashes are actually not wanted in the output and should be removed instead.