Change @PATH to @DIR for clarity. Add @FILENAME. Addresses #869.

@DIR expands to directory path of the script, @FILENAME expands to just
the script file name without path.

testing/btest/Baseline/language.at-dir/out

@@ -0,0 +1 @@
+/Users/jsiwek/Projects/bro/bro/testing/btest/.tmp/language.at-dir

testing/btest/Baseline/language.at-filename/out

@@ -0,0 +1 @@
+at-filename.bro

testing/btest/Baseline/language.at-path/out

@@ -1 +0,0 @@
-/Users/jsiwek/Projects/bro/bro/testing/btest/.tmp/language.at-path

testing/btest/language/at-dir.bro

@@ -3,8 +3,8 @@
 # @TEST-EXEC: bro -b ./pathtest.bro >out2
 # @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff out2
 
-print @PATH;
+print @DIR;
 
 @TEST-START-FILE pathtest.bro
-print @PATH;
+print @DIR;
 @TEST-END-FILE

testing/btest/language/at-filename.bro

@@ -0,0 +1,4 @@
+# @TEST-EXEC: bro -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+print @FILENAME;