practracker: Capture stderr output in test_practracker.sh

And canonicalise file paths, so that output is consistent, even in
out-of-tree builds.

Capturing stderr output produces better diagnostics.

Diagnostics for 32705.

scripts/maint/practracker/testdata/ex0-expected.txt

@@ -1,3 +1,4 @@
+Unusual pattern permitted.h in scripts/maint/practracker/testdata
 problem file-size a.c 41
 problem include-count a.c 6
 problem function-size a.c:i_am_a_function() 9

scripts/maint/practracker/testdata/ex1-expected.txt

@@ -1,3 +1,4 @@
+Unusual pattern permitted.h in scripts/maint/practracker/testdata
 problem function-size a.c:i_am_a_function() 9
 (warning) problem function-size a.c:another_function() 12
 problem function-size b.c:foo() 4

scripts/maint/practracker/testdata/ex1-overbroad-expected.txt

@@ -1,2 +1,3 @@
+Unusual pattern permitted.h in scripts/maint/practracker/testdata
 problem file-size a.c 45 -> 41
 problem file-size z.c 100 -> 0