run-tests: pre instantiate pygments objects

This speeds up run-tests.py diff output by 10x, which affects developer
experience significantly. As demonstrated by the following test:

```
#require pygments

  $ for i in `seq 1 200`; do
  >   echo '  $ echo '$i >> test-a.t
  >   echo '  wrong' >> test-a.t
  > done

  $ cat > walltime.py <<EOF
  > from __future__ import absolute_import, print_function
  > import os, sys, time
  > t1 = time.time()
  > os.system(' '.join(sys.argv[1:]) + ' >/dev/null 2>/dev/null')
  > t2 = time.time()
  > print('%0.2f' % (t2 - t1))
  > EOF

  $ $PYTHON walltime.py $TESTDIR/run-tests.py -l --color=never $TESTTMP/test-a.t
  0.24

Before this patch:

  $ $PYTHON walltime.py $TESTDIR/run-tests.py -l --color=always $TESTTMP/test-a.t
  2.46

After this patch:

  $ $PYTHON walltime.py $TESTDIR/run-tests.py -l --color=always $TESTTMP/test-a.t
  0.25
```

tests/run-tests.py

@@ -96,6 +96,8 b" if os.name != 'nt':"
         import pygments.lexers as lexers
         import pygments.formatters as formatters
         pygmentspresent = True
+        difflexer = lexers.DiffLexer()
+        terminal256formatter = formatters.Terminal256Formatter()
     except ImportError:
         pass
 
@@ -1651,10 +1653,9 b' class TestResult(unittest._TextTestResul'
                     self.stream.write('\n')
                     for line in lines:
                         if self.color:
-                            line = pygments.highlight(
-                                    line,
-                                    lexers.DiffLexer(),
-                                    formatters.Terminal256Formatter())
+                            line = pygments.highlight(line,
+                                                      difflexer,
+                                                      terminal256formatter)
                         if PYTHON3:
                             self.stream.flush()
                             self.stream.buffer.write(line)