sto pusing -toolexec directly in the tests

Since we introduced $GARBLE_DIR, we stopped recommending the use of
toolexec directly. It's still possible to set up the right flags and env
vars, but that will be a moving target.

In particular, string obfuscation in #16 will require using $GARBLE_DIR
in more scenarios. A work-in-progress patch for string obfuscation
triggered this test script to start failing for the reason above.

While at it, we don't care about what the second build contains, since
we already compare it with the previous build.

testdata/scripts/basic.txt

@@ -1,9 +1,9 @@
-# Check that we fail if the user forgot -trimpath.
+# Check that we fail if the user ran with -toolexec but without -trimpath.
 ! exec go build -a -toolexec=garble main.go
 stderr 'should be used alongside -trimpath'
 
 # Check that the simplest use of garble works.
-exec go build -a -trimpath -toolexec=garble main.go
+garble build main.go
 exec ./main
 cmp stderr main.stderr
 
@@ -15,12 +15,10 @@ cmp stderr main.stderr
 
 [short] stop # checking that the build is reproducible is slow
 
-# Finally, check that the 'garble build' shortcut works, and produces the same
-# binary.
+# Also check that the binary is reproducible.
 cp main$exe main_old$exe
 rm main$exe
 garble build main.go
-! binsubstr main$exe 'globalVar'
 bincmp main$exe main_old$exe
 
 # Check that the program works as expected without garble. No need to verify