cmd/gomobile: skip test if keytool is bad
cmd/gomobile: skip test if keytool is bad

Looks like the binary is present on OS X even when Java is not
installed, and it returns an error when called.

Change-Id: Icbd42637f718a4fc05e65ea3973a584d1f24850e
Reviewed-on: https://go-review.googlesource.com/12743
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

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