As per https://golang.org/cmd/go/#hdr-Build_constraints,
using GOOS=android also matches tags and files for GOOS=linux,
so exclude it explicetly to avoid collisions.
Also using GOOS=illumos matches tags and files for GOOS=solaris.
Fixes#843.
This change allows the interpreter to execute tests and benchmarks
functions provided by packages.
The test subcommand is similar to the "go test" command and
all the relevant flags have been kept.
The ability to evaluate a directory or a package has also been added.
A new method Symbol to access exported symbol values of an interpreted
package has been added. This method is used by the test subcommand.
An EvalTest method has been added to evaluate all Go files, including "*_test.go".
The testing packages from the standard library have been added to stdlib used
symbols.
