testdata: reduce the cost of short tests

Reduces "go test -short" with a warm build cache from ~9s to ~4s. The
main offender was the use of "-a" in the "garble test" call; I think I
added that flag to rebuild all packages when debugging an error, and
forgot to remove it.

testdata/scripts/basic.txt

@@ -28,7 +28,7 @@ stdout 'unknown'
 [short] stop # checking that the build is reproducible is slow
 
 # Check that we fail if the user used "go build -toolexec garble" instead of "garble build"
-! go build -a -toolexec=garble main.go
+! go build -toolexec=garble main.go
 stderr 'not running "garble \[command\]"'
 
 # Also check that the binary is reproducible.

testdata/scripts/reverse.txt

@@ -18,6 +18,8 @@ stdin main.stderr
 garble reverse
 cmp stdout reverse.stdout
 
+[short] stop # no need to verify this with -short
+
 # Ensure that the reversed output matches the non-garbled output.
 go build -trimpath
 exec ./main

testdata/scripts/test.txt

@@ -1,5 +1,5 @@
 # build the test binary
-garble test -c -a
+garble test -c
 ! stdout 'PASS'
 binsubstr bar.test$exe 'TestFoo' 'TestSeparateFoo'
 ! binsubstr bar.test$exe 'LocalFoo|ImportedVar|OriginalFuncName'