Fail loudly if new benchmark has no output.

When --make_benchmarks is on, a misconfigured / buggy test program might
fail to produce any output. If this happens, the regression testing
script will continue executing and eventually crash when it can't find
the .status file that it is expecting to be there. This produces an
unintuitive backtrace that does not make clear the root cause of the
problem.

This PR fixes the above by failing with a clear error message as soon as
it can be determined that output from the test program cannot be found.

regtest.py

@@ -48,15 +48,15 @@ def find_build_dirs(tests):
     last_safe = False
 
     for obj in tests:
-        
+
         # keep track of the build directory and which source tree it is
         # in (e.g. the extra build dir)
-        
+
         # first find the list of unique build directories
         dir_pair = (obj.buildDir, obj.extra_build_dir)
         if build_dirs.count(dir_pair) == 0:
             build_dirs.append(dir_pair)
-            
+
         # re-make all problems that specify an extra compile argument,
         # and the test that comes after, just to make sure that any
         # unique build commands are seen.
@@ -66,9 +66,9 @@ def find_build_dirs(tests):
                 obj.reClean = 0
             else:
                 last_safe = True
-                
+
     return build_dirs
-                
+
 def cmake_setup(suite):
     "Setup for cmake"
 
@@ -1177,11 +1177,14 @@ def test_suite(argv):
         # archive (or delete) the output
         #----------------------------------------------------------------------
         suite.log.log("archiving the output...")
+        match_count = 0
         for pfile in os.listdir(output_dir):
 
             if (os.path.isdir(pfile) and
                 re.match(f"{test.name}.*_(plt|chk)[0-9]+", pfile)):
 
+                match_count += 1
+
                 if suite.purge_output == 1 and not pfile == output_file:
 
                     # delete the plt/chk file
@@ -1206,6 +1209,9 @@ def test_suite(argv):
                         except OSError:
                             suite.log.warn(f"unable to remove {pfile}")
 
+        if (match_count == 0):
+            suite.log.fail("ERROR: test output could not be found!")
+
 
         #----------------------------------------------------------------------
         # write the report for this test