Fix --srcpath command line option

The path was prematurely expanded to an absoulte path; later
it was treated as a relative path to "test".

A workaround is to use:

    % JAVA_OPTS=-Dpartest.srcdir=disabled test/partest test/disabled/neg/t7020.scala

    baseDir:             /Users/jason/code/scala/build/pack/lib
    sourceDir:           /Users/jason/code/scala/test/disabled

    Selected 1 tests drawn from specified tests

     starting 1 test in neg
    ok 1 - neg/t7020.scala

    1/1 passed (elapsed time: 00:00:04)
    Test Run PASSED

Which avoids the failure:

    test/partest --srcpath disabled disabled/neg/t7020.scala Discarding 1 invalid test paths
    Uncaught exception on thread Thread[main,5,main]: java.lang.RuntimeException: Directory 'test' not found.
    java.lang.RuntimeException: Directory 'test' not found.

I tested this commit with:

    % (cd /code/scala-partest && sbt 'set publishTo := Some(Resolver.file("file",  new File(Path.userHome.absolutePath+"/.m2/repository")))' publish)
    ...
    [info]  published scala-partest_2.11.0-M5 to /Users/jason/.m2/repository/org/scala-lang/modules/scala-partest_2.11.0-M5/1.0.0-SNAPSHOT/scala-partest_2.11.0-M5-1.0.0-SNAPSHOT.jar

    % git diff versions.properties
    diff --git a/versions.properties b/versions.properties
    index 865ce3e..0dd4369 100644
    --- a/versions.properties
    +++ b/versions.properties
    @@ -2,6 +2,6 @@ starr.version=2.11.0-M5

     # the below is used for depending on dependencies like partest
     scala.binary.version=2.11.0-M5
    -partest.version.number=1.0-RC5
    +partest.version.number=1.0.0-SNAPSHOT
     scala-xml.version.number=1.0-RC4
     scala-parser-combinators.version.number=1.0-RC2
    
    % rm build/pack/partest.properties

    % test/partest --srcpath disabled test/disabled/neg/t7020.scala
    ...
    sourceDir:           /Users/jason/code/scala/test/disabled

    Selected 1 tests drawn from specified tests

    # starting 1 test in neg
    ok 1 - neg/t7020.scala

    1/1 passed (elapsed time: 00:00:04)
    Test Run PASSED