run-tests: make sure to check if pygments is installed before using it

e80041832e introduced support to color the output of tests but used pygments
without checking whether it's installed or not. That breaks test-run-tests.t for
machines which don't have pygments installed. This patch conditionalize the
color test in test-run-tests.t and also add a check to make sure pygments is
installed before using that.

tests/run-tests.py

@@ -89,7 +89,7 b" osenvironb = getattr(os, 'environb', os."
 processlock = threading.Lock()
 
 with_color = False
-
+pygmentspresent = False
 # ANSI color is unsupported prior to Windows 10
 if os.name != 'nt':
     try: # is pygments installed
@@ -97,6 +97,7 b" if os.name != 'nt':"
         import pygments.lexers as lexers
         import pygments.formatters as formatters
         with_color = True
+        pygmentspresent = True
     except ImportError:
         pass
 
@@ -1650,7 +1651,7 b' class TestResult(unittest._TextTestResul'
                 else:
                     self.stream.write('\n')
                     for line in lines:
-                        if with_color:
+                        if with_color and pygmentspresent:
                             line = pygments.highlight(
                                     line,
                                     lexers.DiffLexer(),

tests/test-run-tests.t

@@ -121,7 +121,7 b' test churn with globs'
 
 test diff colorisation
 
-#if no-windows
+#if no-windows pygments
   $ rt test-failure.t --color always
   
   \x1b[38;5;124m--- $TESTTMP/test-failure.t\x1b[39m (esc)