tests: add 'set -x' to the .t sh scripts in run-tests.py debug mode

This makes -d output much more readable when debugging the test framework or
very strange test failures.

tests/run-tests.py

@@ -598,6 +598,8 b' def tsttest(test, wd, options, replaceme'
     f.close()
 
     script = []
+    if options.debug:
+        script.append('set -x\n')
     if os.getenv('MSYSTEM'):
         script.append('alias pwd="pwd -W"\n')
     for n, l in enumerate(t):