E3-920: Set custom E3_MODULES_PATH for test target

The test rule for e3 modules actually does a cellinstall and run the tests inside this temporary installation. The cellinstall needs to overwrite some path variables to work this variables were not accessible through the test rule as they are just changed if cell is on MAKECMDGOALS. This made impossible to use the standard path variables inside module's custom test rules.

To fix it the same mechanism used in cellinstall is implemented for test and the variables paths are now overwritten. This makes possible ie. to use E3_MODULES_INSTALL_LOCATION_BIN inside custom test rules.

This commit also introduces an unwanted behavior. As the test rule is now overwriting the standard path variables it cannot longer be along with the install rule. For example, running make test install will install the module inside the test dir. This is a known issue for cellinstall as well and a restructure of the Makefile code should be made to isolate the rules.