Throw error if no test module

[aciidgh]

When no Test modules are present, running swift test produces this error :

Usage: xctest [-XCTest All | <TestCaseClassName/testMethodName>] <path of unit to be tested>

Failure: No test bundle found at path `/Users/ankit/mycode/temp/.build/debug/Package.xctest`

Displaying process arguments and environment for debugging purposes:
Arguments: (
    "/Applications/Xcode.app/Contents/Developer/usr/bin/xctest",
    "/Users/ankit/mycode/temp/.build/debug/Package.xctest"
)
Environment: {
    HOME = "/Users/ankit";
    PATH = "/Library/Developer/Toolchains/swift-latest.xctoolchain/usr/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin";
    "SPM_INSTALL_PATH" = "/Users/ankit/mycode/temp/.build";
    "__CF_USER_TEXT_ENCODING" = "0x1F5:0x0:0x0";
}

This patch will show this error instead :

error: no tests found to execute, create a test-module in `Tests` directory

by checking if Package.xctest or test-Package exists.

It might be possible to detect if there are no test targets during build phase of tests if in future swift-build-tools reports some error while building an empty target.

[aciidgh]

Squashed, rebased and resolved conflicts 🤓

[mxcl]

@swift-ci Please test

[aciidgh]

@mxcl can we avoid running all the tests in CI?

[mxcl]

@aciidb0mb3r yes @shahmishal is going to make this happen.

[aciidgh]

Cool, also CI used to send in the beginning to the committer but now don't get any notification if when build finishes. Is there a way to subscribe ?