test suite: allow skipping of expensive tests

test suite: allow skipping of expensive tests

Defined a function attribute for KohaTest and subclasses
called 'Expensive'.  When a test method has that attribute,
it is skipped unless the RUN_EXPENSIVE_TESTS environment
variable is true.

To mark a test method, expensive, do this:

sub test_foo : Tests(4) Expensive { ... }

To mark a whole class and its subclasses expensive,
define a SKIP_CLASS sub (with empty body) with the
Expensive attribute:

sub SKIP_CLASS : Expensive { }

Updated the t/Makefile so that 'make test' runs
nonexpensive tests, while 'make fulltest' runs both
cheap and expensive tests.

Marked KohaTest::Installer test class expensive.

Signed-off-by: Andrew Moore <andrew.moore@liblime.com>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>