Go 1.15 Support

[shibumi]

Description of issue or feature request:

In late August Go 1.15 will arrive, I had a look on the changelog and this release will fix a few issues for us.
Full changelog is here: https://tip.golang.org/doc/go1.15

Current behavior + potential impact:

• ecdsa signature: Right now we are creating our own ASN1 ecdsa signature object for signing. With Go 1.15 we can use the new ecdsa functions ecdsa.SignASN1 and ecdsa.VerifyASN1

• Test cleanup: Right now our test directory cleanup is broken and does not work correctly. One way for fixing this has been proposed in: Test clean up does not work due to deferring functions not being called #60 . Go 1.15 reduces a new way: 1. TestMain does not need an exit code anymore. The exit code will be automatically wrapped around the TestMain function. 2. TestMain has now support for creating temporary test directories. This may be easier to use, because TestMain will take care about the clean up. No deferred functions needed anymore. See also:

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The testing.T type now has a Deadline method that reports the time at which the test binary will have exceeded its timeout.
A TestMain function is no longer required to call os.Exit. If a TestMain function returns, the test binary will call os.Exit with the value returned by m.Run. The new methods T.TempDir and B.TempDir return temporary directories that are automatically cleaned up at the end of the test. go test -v now groups output by test name, rather than printing the test name on each line.

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