[Truffle] Tag "slow" core specs.

* Tag all examples of Encoding convpath specs,
  so even the expensive before(:all) is not run.
* Tag specs invoking GC, they are a code smell and slow down the system.
* Tag some eval specs using ruby_exe and some using sleep.
* Before: 33s (37s total)
  After:  17s (21s total)

spec/truffle/tags/core/encoding/converter/convpath_tags.txt

@@ -1 +1,8 @@
 fails:Encoding::Converter#convpath indicates if crlf_newline conversion would occur
+slow:Encoding::Converter#convpath returns an Array
+slow:Encoding::Converter#convpath returns each encoding pair as a sub-Array
+slow:Encoding::Converter#convpath returns each encoding as an Encoding object
+slow:Encoding::Converter#convpath returns multiple encoding pairs when direct conversion is impossible
+slow:Encoding::Converter#convpath sets the last element of each pair to the first element of the next
+slow:Encoding::Converter#convpath only lists a source encoding once
+slow:Encoding::Converter#convpath indicates if crlf_newline conversion would occur

spec/truffle/tags/core/encoding/converter/search_convpath_tags.txt

@@ -2,3 +2,11 @@ fails:Encoding::Converter.search_convpath returns multiple encoding pairs when d
 fails:Encoding::Converter.search_convpath sets the last element of each pair to the first element of the next
 fails:Encoding::Converter.search_convpath indicates if crlf_newline conversion would occur
 fails:Encoding::Converter.search_convpath raises an Encoding::ConverterNotFoundError if no conversion path exists
+slow:Encoding::Converter.search_convpath returns an Array
+slow:Encoding::Converter.search_convpath returns each encoding pair as a sub-Array
+slow:Encoding::Converter.search_convpath returns each encoding as an Encoding object
+slow:Encoding::Converter.search_convpath returns multiple encoding pairs when direct conversion is impossible
+slow:Encoding::Converter.search_convpath sets the last element of each pair to the first element of the next
+slow:Encoding::Converter.search_convpath only lists a source encoding once
+slow:Encoding::Converter.search_convpath indicates if crlf_newline conversion would occur
+slow:Encoding::Converter.search_convpath raises an Encoding::ConverterNotFoundError if no conversion path exists

spec/truffle/tags/core/gc/garbage_collect_tags.txt

@@ -0,0 +1 @@
+slow:GC#garbage_collect always returns nil

spec/truffle/tags/core/gc/start_tags.txt

@@ -0,0 +1 @@
+slow:GC.start always returns nil

spec/truffle/tags/core/kernel/eval_tags.txt

@@ -6,4 +6,7 @@ fails:Kernel#eval sets constants at the toplevel from inside a block
 fails:Kernel#eval uses the filename of the binding if none is provided
 fails:Kernel#eval uses the receiver as self inside the eval
 fails:Kernel#eval returns from the scope calling #eval when evaluating 'return'
-fails:Kernel#eval unwinds through a Proc-style closure and returns from a lambda-style closure in the closure chain
+fails:Kernel#eval unwinds through a Proc-style closure and returns from a lambda-style closure in the closure chain
+slow:Kernel#eval raises a LocalJumpError if there is no lambda-style closure in the chain
+slow:Kernel#eval does not share locals across eval scopes
+

spec/truffle/tags/core/kernel/sleep_tags.txt

@@ -8,3 +8,5 @@ fails:Kernel#sleep pauses execution indefinitely if not given a duration
 fails:Kernel.sleep needs to be reviewed for spec completeness
 fails:Kernel#sleep raises a TypeError when passed nil
 fails:Kernel#sleep raises a TypeError when passed a String
+slow:Kernel#sleep accepts a Float
+slow:Kernel#sleep accepts a Rational

spec/truffle/tags/core/objectspace/garbage_collect_tags.txt

@@ -0,0 +1,4 @@
+slow:ObjectSpace.garbage_collect can be invoked without any exceptions
+slow:ObjectSpace.garbage_collect doesn't accept any arguments
+slow:ObjectSpace.garbage_collect ignores the supplied block
+slow:ObjectSpace.garbage_collect always returns nil