Allow additional options for nar-testCompile phase

Provide a second method to extend the default CompilerDef and LinkerDef
argument lists with a new 'testOptions' collection. Currently these
options are additive only, and are appended to the standard
configuration.

Test-specific compilation options are useful e.g. for instrumentation of
test code for coverage purposes (such as `-fprofile-arcs -ftest-coverage`
for gcov), or for enabling debug macros in test executions.
Add unit test for testOptions compilation flags

Add a very simple C unit test class which verifies the testOptions
compiler arguments.

Note that in its current implementation the testOptions do not apply to
the main source code.
Verify that <testOptions> are inactive for src/main/c/

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'TestCompileOptions'

This branch provides a second method to extend the default CompilerDef
and LinkerDef argument lists with a new 'testOptions' collection.

It is a slightly enhanced version of
https://github.com/maven-nar/nar-maven-plugin/pull/92.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>