valgrind suppressions: Suppress libmagic leaks.
is_lv: Does not need to depend on lvm2.

It only uses 'stat', not any lvm binaries.
tests: 'If' and 'Unless' conditions never used, so remove them.
tests: More flexible re-implementation of C API tests.

Instead of using the various 'TestOutput', 'TestOutputList' etc
macros, it makes better sense to let the tests contain fragments of C
code to perform the checks.

Several new macros are added:

- 'TestResult': This macro takes a C expression which is evaluated to
test the result of commands.  For example to compare if a string
result has some value:

   TestResult (* command sequence *), "STREQ (ret, \"abcd\")"

The variable "ret" contains the result from the last command in the
sequence.  But also, variables "ret1", "ret2", etc contain the results
from the last but one command, the command before that, and so forth,
allowing much more flexible tests to be written.

- 'TestResultTrue', 'TestResultFalse': Wrappers that test the last
command for true/false.
tests: Replace TestOutputTrue/False with TestResultTrue/False.
tests: Replace TestOutputInt{,Op} with TestResult.
tests: Replace TestOutputStruct with TestResult.
tests: Replace TestOutputList{,OfDevices} with TestResult.

This adds helper C functions 'is_string_list' and 'is_device_list'
allowing these tests to be carried out in generic C code instead of as
specialized tests.
tests: Replace TestOutput/TestOutputDevice with TestResult{String,Device}.
tests: Remove TestOutputLength (not used by any tests).
tests: Replace TestOutputFileMD5 with TestResult and C test.
tests: Replace TestOutputBuffer with TestResult and C test.
tests: Replace TestOutputHashtable with TestResult and C test.
tests: Split C API tests into regular C file + generated tests file.