Add singular flag to exclude all tests and test requirements

[Conan-Kudo]

As I've been working on #33, I've encountered an issue attempting to build it with tests excluded.

Snippet of output from the mock build:

+ gradle-local installDist --offline -x test -x testAll -x testAllButLongCrypto -x testAllLong -x testGlobal -x testScenario

FAILURE: Build failed with an exception.

* Where:
Build file '/builddir/build/BUILD/syncany-0.4.6-alpha/build.gradle' line: 29

* What went wrong:
A problem occurred evaluating root project 'syncany-0.4.6-alpha'.
> Could not resolve all dependencies for configuration 'classpath'.
   > Could not resolve net.saliman:gradle-cobertura-plugin:2.2.2.
     Required by:
         unspecified:unspecified:unspecified
      > No cached version of net.saliman:gradle-cobertura-plugin:2.2.2 available for offline mode.
   > Could not resolve org.kt3k.gradle.plugin:coveralls-gradle-plugin:2.3.1.
     Required by:
         unspecified:unspecified:unspecified
      > No cached version of org.kt3k.gradle.plugin:coveralls-gradle-plugin:2.3.1 available for offline mode.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 5.23 secs

I don't really want to go through and package all the test dependencies right now, and it's okay to have packages that have tests disabled. What I'd like to have is a singular switch that disables all tests, test report generators, and all dependencies related to tests.

[binwiederhier]

Something like ./gradlew -d noTests should do.
Could be implemented in build.gradle like this:

if (not noTests is set) {
   apply from: 'gradle/gradle/reports.coverage.gradle'
   apply from: 'gradle/gradle/reports.tests.gradle'
   apply from: 'gradle/gradle/reports.javadoc.gradle'
   apply from: 'gradle/gradle/reports.sonarqube.gradle'
   ...
}

[Conan-Kudo]

Well, I still want Javadoc information to be generated, since that's useful for others to write things to use Syncany. I can satisfy the dependencies for the Javadoc generator.

[binwiederhier]

@Conan-Kudo Any luck with this flag?