Add test creating multiple plugins with load dependencies.

If we load plugins purely alphabetically, the 1st Zeek run in the test
will success while the 2nd will fail.

testing/btest/plugins/plugin-load-dependency.zeek

@@ -0,0 +1,31 @@
+# @TEST-EXEC: mkdir 1
+# @TEST-EXEC: cd 1 && ${DIST}/auxil/zeek-aux/plugin-support/init-plugin -u . Testing Plugin1
+# @TEST-EXEC: cp -r %DIR/plugin-load-dependency/1 .
+# @TEST-EXEC: cd 1 && ./configure --zeek-dist=${DIST} && make
+
+# @TEST-EXEC: mkdir 2
+# @TEST-EXEC: cd 2 && ${DIST}/auxil/zeek-aux/plugin-support/init-plugin -u . Testing Plugin2
+# @TEST-EXEC: cp -r %DIR/plugin-load-dependency/2 .
+# @TEST-EXEC: cd 2 && ./configure --zeek-dist=${DIST} && make
+
+# @TEST-EXEC: mkdir 3
+# @TEST-EXEC: cd 3 && ${DIST}/auxil/zeek-aux/plugin-support/init-plugin -u . Testing Plugin3
+# @TEST-EXEC: cp -r %DIR/plugin-load-dependency/3 .
+# @TEST-EXEC: cd 3 && ./configure --zeek-dist=${DIST} && make
+
+# The following run will only work if Zeek loads plugin2 before plugin3 (which
+# by alphabetical loading will be the case)
+# @TEST-EXEC: ZEEK_PLUGIN_PATH=. zeek -b -N Testing::Plugin3 Testing::Plugin2 | grep -v Zeek:: | sort >> output
+#
+# @TEST-EXEC: echo >>output
+#
+# The following run will only work if Zeek loads plugin2 before plugin1 (which
+# by alphabetical loading will not be the case).
+# @TEST-EXEC: ZEEK_PLUGIN_PATH=. zeek -b -N Testing::Plugin1 Testing::Plugin2 | grep -v Zeek:: | sort >> output
+#
+# @TEST-EXEC: echo >>output
+#
+# Finally, try it with self-discovery of all three plugins too.
+# @TEST-EXEC: ZEEK_PLUGIN_PATH=. zeek -N | grep -v Zeek:: | sort >> output
+#
+# @TEST-EXEC: btest-diff output