winpr-env: extended GetSetEB ctest

Check if Get function only returns values for the exact variable name
winpr-env: fix in GetEnvironmentVariableEBA

GetEnvironmentVariableEBA didn't do exact matching of the requested variable name. If a variable
with similar but shorter name was set in the environment block it was returned.
For example if HOME was set in the environment block and HOMEX was requested the value of HOME was
returned.